An architectural perspective on signaling by the pre-, alphabeta and gammadelta T cell receptors

Development of γδ T cells in the thymus - A human perspective

γδ T cells are increasingly emerging as crucial immune regulators that can take on innate and adaptive roles in the defence against pathogens. Although they arise within the thymus from the same hematopoietic precursors as conventional αβ T cells, the development of γδ T cells is less well understood. In this review, we focus on summarising the current state of knowledge about the cellular and molecular processes involved in the generation of γδ T cells in human.

Application of double-negative T cells in haematological malignancies: recent progress and future directions

Haematologic malignancies account for a large proportion of cancers worldwide. The high occurrence and mortality of haematologic malignancies create a heavy social burden. Allogeneic haematopoietic stem cell transplantation is widely used in the treatment of haematologic malignancies. However, graft-versus-host disease and relapse after allogeneic haematopoietic stem cell transplantation are inevitable. An emerging treatment method, adoptive cellular therapy, has been effectively used in the treatment of haematologic malignancies. T cells, natural killer (NK) cells and tumour-infiltrating lymphocytes (TILs) all have great potential in therapeutic applications, and chimeric antigen receptor T (CAR-T) cell therapy especially has potential, but cytokine release syndrome and off-target effects are common. Efficient anticancer measures are urgently needed. In recent years, double-negative T cells (CD3⁺CD4^-CD8^-) have been found to have great potential in preventing allograft/xenograft rejection and inhibiting graft-versus-host disease. They also have substantial ability to kill various cell lines derived from haematologic malignancies in an MHC-unrestricted manner. In addition, healthy donor expanded double-negative T cells retain their antitumour abilities and ability to inhibit graft-versus-host disease after cryopreservation under good manufacturing practice (GMP) conditions, indicating that double-negative T cells may be able to be used as an off-the-shelf product. In this review, we shed light on the potential therapeutic ability of double-negative T cells in treating haematologic malignancies. We hope to exploit these cells as a novel therapy for haematologic malignancies.

γδ T cells in artiodactyls: Focus on swine

Vaccination is the most effective medical strategy for disease prevention but there is a need to improve livestock vaccine efficacy. Understanding the structure of the immune system of swine, which are considered a γδ T cell "high" species, and thus, particularly how to engage their γδ T cells for immune responses, may allow for development of vaccine optimization strategies. The propensity of γδ T cells to home to specific tissues, secrete pro-inflammatory and regulatory cytokines, exhibit memory or recall responses and even function as antigen-presenting cells for αβ T cells supports the concept that they have enormous potential for priming by next generation vaccine constructs to contribute to protective immunity. γδ T cells exhibit several innate-like antigen recognition properties including the ability to recognize antigen in the absence of presentation via major histocompatibility complex (MHC) molecules enabling γδ T cells to recognize an array of peptides but also non-peptide antigens in a T cell receptor-dependent manner. γδ T cell subpopulations in ruminants and swine can be distinguished based on differential expression of the hybrid co-receptor and pattern recognition receptors (PRR) known as workshop cluster 1 (WC1). Expression of various PRR and other innate-like immune receptors diversifies the antigen recognition potential of γδ T cells. Finally, γδ T cells in livestock are potent producers of critical master regulator cytokines such as interferon (IFN)-γ and interleukin (IL)-17, whose production orchestrates downstream cytokine and chemokine production by other cells, thereby shaping the immune response as a whole. Our knowledge of the biology, receptor expression and response to infectious diseases by swine γδ T cells is reviewed here.

Wiskott-Aldrich Syndrome Protein: Roles in Signal Transduction in T Cells

Signal transduction regulates the proper function of T cells in an immune response. Upon binding to its specific ligand associated with major histocompatibility complex (MHC) molecules on an antigen presenting cell, the T cell receptor (TCR) initiates intracellular signaling that leads to extensive actin polymerization. Wiskott-Aldrich syndrome protein (WASp) is one of the actin nucleation factors that is recruited to TCR microclusters, where it is activated and regulates actin network formation. Here we highlight the research that has focused on WASp-deficient T cells from both human and mice in TCR-mediated signal transduction. We discuss the role of WASp in proximal TCR signaling as well as in the Ras/Rac-MAPK (mitogen-activated protein kinase), PKC (protein kinase C) and Ca²⁺-mediated signaling pathways.

Molecular and cellular characterization of European sea bass CD3ε+ T lymphocytes and their modulation by microalgal feed supplementation

The CD3 coreceptor is a master T cell surface marker, and genes encoding CD3ζ, γδ, and ε chains have been reported in several teleost fish. Here, a complete cDNA sequence of CD3ɛ chain was identified from a sea bass (Dicentrarchus labrax L.) gill transcriptome. Its basal expression was quantified in both lymphoid and non-lymphoid organs of sea bass juveniles with real-time qPCR analysis. After either in vitro stimulation of head kidney leukocytes with the T-cell mitogen phytohaemagglutinin or in vivo stimulation with an orally administered Vibrio anguillarum vaccine, CD3ε expression levels increased in head kidney leukocytes, confirming that CD3ε T cells may play important roles in fish systemic protection against pathogens. Further, three peptides were designed on the CD3ɛ cytoplasmic tail region and employed as immunogens for antibody production in rabbit. One antiserum so obtained, named RACD3/1, immunostained a band of the expected size in a western blot of a sea bass thymocyte lysate. The distribution of CD3ε⁺ lymphocyte population in the lymphoid organs and mucosal tissues was addressed in healthy fish by IHC. In decreasing percentage order, CD3ε⁺ lymphocytes were detected by flow cytometry in thymus, peripheral blood leukocytes, gills, head kidney, gut, and spleen. Finally, a significant in vivo enhancement of CD3ε⁺ T intestinal lymphocytes was found in fish fed on diets in which 100% fish meal was replaced by the microalgae Nannochloropsis sp. biomass. These results indicate that CD3ε⁺ T cells are involved in nutritional immune responses.

CFTR is a negative regulator of γδ T cell IFN-γ production and antitumor immunity

CFTR, a chloride channel and ion channel regulator studied mostly in epithelial cells, has been reported to participate in immune regulation and likely affect the risk of cancer development. However, little is known about the effects of CFTR on the differentiation and function of γδ T cells. In this study, we observed that CFTR was functionally expressed on the cell surface of γδ T cells. Genetic deletion and pharmacological inhibition of CFTR both increased IFN-γ release by peripheral γδ T cells and potentiated the cytolytic activity of these cells against tumor cells both in vitro and in vivo. Interestingly, the molecular mechanisms underlying the regulation of γδ T cell IFN-γ production by CFTR were either TCR dependent or related to Ca²⁺ influx. CFTR was recruited to TCR immunological synapses and attenuated Lck-P38 MAPK-c-Jun signaling. In addition, CFTR was found to modulate TCR-induced Ca²⁺ influx and membrane potential (V_m)-induced Ca²⁺ influx and subsequently regulate the calcineurin-NFATc1 signaling pathway in γδ T cells. Thus, CFTR serves as a negative regulator of IFN-γ production in γδ T cells and the function of these cells in antitumor immunity. Our investigation suggests that modification of the CFTR activity of γδ T cells may be a potential immunotherapeutic strategy for cancer.

γδ T Cell Update: Adaptate Orchestrators of Immune Surveillance

As interest in γδ T cells grows rapidly, what key points are emerging, and where is caution warranted? γδ T cells fulfill critical functions, as reflected in associations with vaccine responsiveness and cancer survival in humans and ever more phenotypes of γδ T cell-deficient mice, including basic physiological deficiencies. Such phenotypes reflect activities of distinct γδ T cell subsets, whose origins offer interesting insights into lymphocyte development but whose variable evolutionary conservation can obfuscate translation of knowledge from mice to humans. By contrast, an emerging and conserved feature of γδ T cells is their "adaptate" biology: an integration of adaptive clonally-restricted specificities, innate tissue-sensing, and unconventional recall responses that collectively strengthen host resistance to myriad challenges. Central to adaptate biology are butyrophilins and other γδ cell regulators, the study of which should greatly enhance our understanding of tissue immunogenicity and immunosurveillance and guide intensifying clinical interest in γδ cells and other unconventional lymphocytes.

A comparative view on vitamin C effects on αβ- versus γδ T-cell activation and differentiation

Vitamin C (VitC) is an essential vitamin that needs to be provided through exogenous sources. It is a potent anti-oxidant, and an essential cofactor for many enzymes including a group of enzymes that modulate epigenetic regulation of gene expression. Moreover, VitC has a significant influence on T-cell differentiation, and can directly interfere with T-cell signaling. Conventional CD4 and CD8 T cells express the αβ TCR and recognize peptide antigens in the context of MHC presentation. The numerically small population of γδ T cells recognizes antigens in an MHC-independent manner. γδ T cells kill a broad variety of malignant cells, and because of their unique features, are interesting candidates for cancer immunotherapy. In this review, we summarize what is known about the influence of VitC on T-cell activation and differentiation with a special focus on γδ T cells. The known mechanisms of action of VitC on αβ T cells are discussed and extrapolated to the effects observed on γδ T-cell activation and differentiation. Overall, VitC enhances proliferation and effector functions of γδ T cells and thus may help to increase the efficacy of γδ T cells applied as cancer immunotherapy in adoptive cell transfer.

Ancient Use of Ig Variable Domains Contributes Significantly to the TCRδ Repertoire

The loci encoding B and T cell Ag receptors are generally distinct in commonly studied mammals, with each receptor's gene segments limited to intralocus, cis chromosomal rearrangements. The nurse shark (Ginglymostoma cirratum) represents the oldest vertebrate class, the cartilaginous fish, with adaptive immunity provided via Ig and TCR lineages, and is one species among a growing number of taxa employing Ig-TCRδ rearrangements that blend these distinct lineages. Analysis of the nurse shark Ig-TCRδ repertoire found that these rearrangements possess CDR3 characteristics highly similar to canonical TCRδ rearrangements. Furthermore, the Ig-TCRδ rearrangements are expressed with TCRγ, canonically found in the TCRδ heterodimer. We also quantified BCR and TCR transcripts in the thymus for BCR (IgHV-IgHC), chimeric (IgHV-TCRδC), and canonical (TCRδV-TCRδC) transcripts, finding equivalent expression levels in both thymus and spleen. We also characterized the nurse shark TCRαδ locus with a targeted bacterial artifical chromosome sequencing approach and found that the TCRδ locus houses a complex of V segments from multiple lineages. An IgH-like V segment, nestled within the nurse shark TCRδ translocus, grouped with IgHV-like rearrangements we found expressed with TCRδ (but not IgH) rearrangements in our phylogenetic analysis. This distinct lineage of TCRδ-associated IgH-like V segments was termed "TAILVs." Our data illustrate a dynamic TCRδ repertoire employing TCRδVs, NARTCRVs, bona fide trans-rearrangements from shark IgH clusters, and a novel lineage in the TCRδ-associated Ig-like V segments.

The γδTCR combines innate immunity with adaptive immunity by utilizing spatially distinct regions for agonist selection and antigen responsiveness

T lymphocytes expressing γδ T cell antigen receptors (TCRs) comprise evolutionarily conserved cells with paradoxical features. On the one hand, clonally expanded γδ T cells with unique specificities typify adaptive immunity. Conversely, large compartments of γδTCR+ intraepithelial lymphocytes (γδ IELs) exhibit limited TCR diversity and effect rapid, innate-like tissue surveillance. The development of several γδ IEL compartments depends on epithelial expression of genes encoding butyrophilin-like (Btnl (mouse) or BTNL (human)) members of the B7 superfamily of T cell co-stimulators. Here we found that responsiveness to Btnl or BTNL proteins was mediated by germline-encoded motifs within the cognate TCR variable γ-chains (Vγ chains) of mouse and human γδ IELs. This was in contrast to diverse antigen recognition by clonally restricted complementarity-determining regions CDR1-CDR3 of the same γδTCRs. Hence, the γδTCR intrinsically combines innate immunity and adaptive immunity by using spatially distinct regions to discriminate non-clonal agonist-selecting elements from clone-specific ligands. The broader implications for antigen-receptor biology are considered.

Id3 Restricts γδ NKT Cell Expansion by Controlling Egr2 and c-Myc Activity

γδ NKT cells are neonatal-derived γδ T lymphocytes that are grouped together with invariant NKT cells based on their shared innate-like developmental program characterized by the transcription factor PLZF (promyelocytic leukemia zinc finger). Previous studies have demonstrated that the population size of γδ NKT cells is tightly controlled by Id3-mediated inhibition of E-protein activity in mice. However, how E proteins promote γδ NKT cell development and expansion remains to be determined. In this study, we report that the transcription factor Egr2, which also activates PLZF expression in invariant NKT cells, is essential for regulating γδ NKT cell expansion. We observed a higher expression of Egr family genes in γδ NKT cells compared with the conventional γδ T cell population. Loss of function of Id3 caused an expansion of γδ NKT cells, which is accompanied by further upregulation of Egr family genes as well as PLZF. Deletion of Egr2 in Id3-deficient γδ NKT cells prevented cell expansion and blocked PLZF upregulation. We further show that this Egr2-mediated γδ NKT cell expansion is dependent on c-Myc. c-Myc knockdown attenuated the proliferation of Id3-deficient γδ NKT cells, whereas c-Myc overexpression enhanced the proliferation of Id3/Egr2-double-deficient γδ NKT cells. Therefore, our data reveal a regulatory circuit involving Egr2-Id3-E2A, which normally restricts the population size of γδ NKT cells by adjusting Egr2 dosage and c-Myc expression.

Peculiar Expression of CD3-Epsilon in Kidney of Ginbuna Crucian Carp

TCR/CD3 complex is composed of the disulfide-linked TCR-αβ heterodimer that recognizes the antigen as a peptide presented by the MHC, and non-covalently paired CD3γε- and δε-chains together with disulfide-linked ζ-chain homodimers. The CD3 chains play key roles in T cell development and T cell activation. In the present study, we found nor or extremely lower expression of CD3ε in head- and trunk-kidney lymphocytes by flow cytometric analysis, while CD3ε was expressed at the normal level in lymphocytes from thymus, spleen, intestine, gill, and peripheral blood. Furthermore, CD4-1⁺ and CD8α⁺ T cells from kidney express Zap-70, but not CD3ε, while the T cells from other tissues express both Zap-70 and CD3ε, although expression of CD3ε was low. Quantitative analysis of mRNA expression revealed that the expression level of T cell-related genes including tcrb, cd3ε, zap-70, and lck in CD4-1⁺ and CD8α⁺ T cells was not different between kidney and spleen. Western blot analysis showed that CD3ε band was detected in the cell lysates of spleen but not kidney. To be interested, CD3ε-positive cells greatly increased after 24 h in in vitro culture of kidney leukocytes. Furthermore, expression of CD3ε in both transferred kidney and spleen leukocytes was not detected or very low in kidney, while both leukocytes expressed CD3ε at normal level in spleen when kidney and spleen leukocytes were injected into the isogeneic recipient. Lower expression of CD3ε was also found in kidney T lymphocytes of goldfish and carp. These results indicate that kidney lymphocytes express no or lower level of CD3ε protein in the kidney, although the mRNA of the gene was expressed. Here, we discuss this phenomenon from the point of function of kidney as reservoir for T lymphocytes in teleost, which lacks lymph node and bone marrow.

T-cell receptors modify neuronal function in the central nervous system

Recent published findings have shown that many proteins discovered in the immune system and residing on immune cells with well established immune-related functions are also found in neurons of the central nervous system. These studies have uncovered a rich variety of neuronal functions attributed to these immune proteins. This review will focus on two key interacting protein complexes that previously were known for adaptive immune reactions, the major histocompatability complex and the T-cell receptor complex. We will review where these immune proteins are expressed in the CNS and their neuronal function.

Origins of γδ T cell effector subsets: a riddle wrapped in an enigma

αβ and γδ T cells are thought to arise from a common precursor in the thymus but play distinct roles in pathogen resistance. Although conventional αβ T cells exit the thymus in a naive state and acquire effector function in the periphery, the effector fate of many γδ T cells is specified in the thymus and exhibits limited plasticity thereafter. This review describes the current models that have been proposed to explain the acquisition of effector fate by γδ T cells, as well as the apparent linkage to Vγ gene usage. The two predominant models are the predetermination model, which suggests that effector fate is determined prior to TCR expression, perhaps in association with the developmental timing of Vγ rearrangement, and the TCR-dependence model, which proposes that the nature of the TCR signal, particularly its intensity or duration, plays an important role in influencing effector fate.

Immune response of γδT cells in Schistosome japonicum-infected C57BL/6 mouse liver

Systematic evaluation of the role of γδT cells during the Schistosoma japonicum infection has not been reported, despite the fact that γδT cells contribute to many infectious diseases in innate immunity. Therefore, the aim of this study was to observe the properties of γδT cells in the liver of C57BL/6 mice infected by S. japonicum. In this report, using immuno-fluorescent histological analysis, γδT cells were found around hepatic granulomatous. Moreover, the flow cytometry results revealed that the percentage of hepatic γδT cells increased significantly after S. japonicum infection. More interestingly, a subset of CD3(-)γδTCR(+) cells were found and markedly increased after infection. Furthermore, expression of activation markers (CD25 and CD69) and cytokine profiles were detected in these hepatic CD3(+)γδTCR(+) and CD3(-)γδTCR(+) cells. The significantly higher level of CD69, IL-4 and IL-17 were observed in CD3(+)γδTCR(+) cells after infection, suggesting that CD3(+)γδTCR(+) cells instead of CD3(-)γδTCR(+) cells might play a predominant role during the infection. Finally, our results indicated that the expression of NKG2D on CD3(+)γδTCR(+) cells was higher than that on CD3(-)γδTCR(+) cells. Collectively, γδT cells could play an important role in the liver of C57BL/6 mouse during japonicum infection.

Five Layers of Receptor Signaling in γδ T-Cell Differentiation and Activation

The contributions of γδ T-cells to immunity to infection or tumors critically depend on their activation and differentiation into effectors capable of secreting cytokines and killing infected or transformed cells. These processes are molecularly controlled by surface receptors that capture key extracellular cues and convey downstream intracellular signals that regulate γδ T-cell physiology. The understanding of how environmental signals are integrated by γδ T-cells is critical for their manipulation in clinical settings. Here, we discuss how different classes of surface receptors impact on human and murine γδ T-cell differentiation, activation, and expansion. In particular, we review the role of five receptor types: the T-cell receptor (TCR), costimulatory receptors, cytokine receptors, NK receptors, and inhibitory receptors. Some of the key players are the costimulatory receptors CD27 and CD28, which differentially impact on pro-inflammatory subsets of γδ T-cells; the cytokine receptors IL-2R, IL-7R, and IL-15R, which drive functional differentiation and expansion of γδ T-cells; the NK receptor NKG2D and its contribution to γδ T-cell cytotoxicity; and the inhibitory receptors PD-1 and BTLA that control γδ T-cell homeostasis. We discuss these and other receptors in the context of a five-step model of receptor signaling in γδ T-cell differentiation and activation, and discuss its implications for the manipulation of γδ T-cells in immunotherapy.

Expanding roles for ThPOK in thymic development

The role of the zinc finger transcription factor ThPOK (T-helper-inducing POZ-Kruppel-like factor) in promoting commitment of αβ T cells to the CD4 lineage is now well established. New results indicate that ThPOK is also important for the development and/or acquisition of effector functions by other T cell subsets, including several not marked by CD4 expression, i.e. double-negative invariant natural killer T (iNKT) cells, γδ cells, and even memory CD8(+) T cells. There is compelling evidence that ThPOK expression in most or all of these cases is dependent on T-cell receptor signaling and that differences in relative TCR signal strength/length may induce different levels of ThPOK expression. The developmental consequences of ThPOK expression vary according to cell type, which may partly reflect differences in ThPOK levels and/or in transcriptional networks between cell types.

A transcription factor map as revealed by a genome-wide gene expression analysis of whole-blood mRNA transcriptome in multiple sclerosis

Background

Several lines of evidence suggest that transcription factors are involved in the pathogenesis of Multiple Sclerosis (MS) but complete mapping of the whole network has been elusive. One of the reasons is that there are several clinical subtypes of MS and transcription factors that may be involved in one subtype may not be in others. We investigate the possibility that this network could be mapped using microarray technologies and contemporary bioinformatics methods on a dataset derived from whole blood in 99 untreated MS patients (36 Relapse Remitting MS, 43 Primary Progressive MS, and 20 Secondary Progressive MS) and 45 age-matched healthy controls.

Methodology/Principal Findings

We have used two different analytical methodologies: a non-standard differential expression analysis and a differential co-expression analysis, which have converged on a significant number of regulatory motifs that are statistically overrepresented in genes that are either differentially expressed (or differentially co-expressed) in cases and controls (e.g., V$KROX_Q6, p-value <3.31E-6; V$CREBP1_Q2, p-value <9.93E-6, V$YY1_02, p-value <1.65E-5).

Conclusions/Significance

Our analysis uncovered a network of transcription factors that potentially dysregulate several genes in MS or one or more of its disease subtypes. The most significant transcription factor motifs were for the Early Growth Response EGR/KROX family, ATF2, YY1 (Yin and Yang 1), E2F-1/DP-1 and E2F-4/DP-2 heterodimers, SOX5, and CREB and ATF families. These transcription factors are involved in early T-lymphocyte specification and commitment as well as in oligodendrocyte dedifferentiation and development, both pathways that have significant biological plausibility in MS causation.

Distinctive CD3 heterodimeric ectodomain topologies maximize antigen-triggered activation of alpha beta T cell receptors

The alphabeta TCR has recently been suggested to function as an anisotropic mechanosensor during immune surveillance, converting mechanical energy into a biochemical signal upon specific peptide/MHC ligation of the alphabeta clonotype. The heterodimeric CD3epsilongamma and CD3epsilondelta subunits, each composed of two Ig-like ectodomains, form unique side-to-side hydrophobic interfaces involving their paired G-strands, rigid connectors to their respective transmembrane segments. Those dimers are laterally disposed relative to the alphabeta heterodimer within the TCR complex. In this paper, using structure-guided mutational analysis, we investigate the functional consequences of a striking asymmetry in CD3gamma and CD3delta G-strand geometries impacting ectodomain shape. The uniquely kinked conformation of the CD3gamma G-strand is crucial for maximizing Ag-triggered TCR activation and surface TCR assembly/expression, offering a geometry to accommodate juxtaposition of CD3gamma and TCR beta ectodomains and foster quaternary change that cannot be replaced by the isologous CD3delta subunit's extracellular region. TCRbeta and CD3 subunit protein sequence analyses among Gnathostomata species show that the Cbeta FG loop and CD3gamma subunit coevolved, consistent with this notion. Furthermore, restoration of T cell activation and development in CD3gamma(-/-) mouse T lineage cells by interspecies replacement can be rationalized from structural insights on the topology of chimeric mouse/human CD3epsilondelta dimers. Most importantly, our findings imply that CD3gamma and CD3delta evolved from a common precursor gene to optimize peptide/MHC-triggered alphabeta TCR activation.

Determining γδ versus αß T cell development

The thymus produces several types of functionally distinct T cell subsets. However, at a more fundamental level only two genetically distinct T cell lineages exist: the γδ and αß T cell lineages. Precisely how these two T cell lineages are generated from common thymocyte progenitor cells remains to be fully elucidated and is under intense investigation. Here, we highlight recent findings that have helped to provide important clues to the mechanisms that underpin the generation of γδ T cells in the mouse thymus.

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.

Beyond alphabeta/gammadelta lineage commitment: TCR signal strength regulates gammadelta T cell maturation and effector fate

Signaling by the gammadelta T cell receptor (TCR) is required not only for alphabeta/gammadelta lineage commitment but also to activate and elicit effector functions in mature gammadelta T cells. Notably, at both of these stages, the signal delivered by the gammadeltaTCR is more robust than the one delivered by either the preTCR or the alphabetaTCR. Recent studies now provide evidence that signaling by the gammadeltaTCR is also required at other stages during gammadelta T cell development. Remarkably, the strength of the gammadeltaTCR signal also plays a role at these other stages, as evidenced by the findings that genetic manipulation of gammadeltaTCR signal strength affects gammadelta T cell maturation and effector fate. In this review, we discuss how a strong TCR signal is a recurring theme in gammadelta T cell development and activation.

TCR-mediated ThPOK induction promotes development of mature (CD24-) gammadelta thymocytes

T lymphocytes develop into two major lineages characterized by expression of the alphabeta and gammadelta T cell receptor (TCR) heterodimers. Within each major lineage, further specialization occurs, resulting in distinct subsets that differ in TCR specificity, phenotype and functional attributes. Thus, in the murine thymus, two distinct subsets of mature (CD24-) gammadelta cells have been identified, that is NK1.1+ cells, which are enriched for Vgamma1.1 usage and selectively produce IFNgamma on stimulation, and CCR6+ cells, which are enriched for Vgamma2 usage produce IL17. The upstream signals and transcriptional pathways that promote development of these distinct gammadelta subsets remain relatively poorly understood. Here, we show that the Zn-finger transcription factor ThPOK has a critical function in the development of gammadelta thymocytes. Thus, lack of functional ThPOK causes a marked reduction in the percentage and absolute number of mature gammadelta thymocytes, and a particularly severe reduction of NK1.1+ cells. Conversely, constitutive ThPOK expression leads to a striking increase in mature NK1.1+ gammadelta thymocytes. Further, we show that ThPOK induction in gammadelta thymocytes is induced by strong TCR signals mediated by engagement with antibody or high-affinity endogenous ligands, and that an important ThPOK cis-acting element, the distal regulatory element (DRE), is sufficient for this TCR-dependent induction. These results show that ThPOK expression in gammadelta thymocytes is regulated in part by the strength of TCR signalling, identify ThPOK as an important mediator of gammadelta T cell development/maturation, and lend strong support to the view that development of a significant fraction of gammadelta T cells depends on TCR engagement/signalling.

ITAM-mediated signaling by the T-cell antigen receptor

Signal transduction by the T-cell antigen receptor (TCR) is initiated by phosphorylation of conserved motifs (ITAMs) contained within the cytoplasmic domains of the invariant subunits. TCR complexes contain a total of 10 ITAMs and this unusual configuration has prompted studies of the role of specific ITAMs, or of ITAM multiplicity, in regulating TCR-directed developmental and effector responses. Here, we summarize data generated during the past two decades and discuss how these findings have in some cases resolved, and in others complicated, outstanding questions relating to the function of TCR ITAMs.

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

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

Structural and functional evidence that Nck interaction with CD3epsilon regulates T-cell receptor activity

Recruitment of signaling molecules to the cytoplasmic domains of the CD3 subunits of the T-cell receptor (TCR) is crucial for early T-cell activation. These transient associations either do or do not require tyrosine phosphorylation of CD3 immune tyrosine activation motifs (ITAMs). Here we show that the non-ITAM-requiring adaptor protein Nck forms a complex with an atypical PxxDY motif of the CD3epsilon tail, which encompasses Tyr166 within the ITAM and a TCR endocytosis signal. As suggested by the structure of the complex, we find that Nck binding inhibits phosphorylation of the CD3epsilon ITAM by Fyn and Lck kinases in vitro. Moreover, the CD3epsilon-Nck interaction downregulates TCR surface expression upon physiological stimulation in mouse primary lymph node cells. This indicates that Nck performs an important regulatory function in T lymphocytes by inhibiting ITAM phosphorylation and/or removing cell surface TCR via CD3epsilon interaction.

T-cell lymphomas in T-cell-specific Pten-deficient mice originate in the thymus

Phosphatase and tensin homolog deleted on chromosome 10 (Pten) is a tumor suppressor protein whose loss of lipid phosphatase activity is associated with lymphomagenesis. We made use of the Cre-loxP system to delete Pten expression in Lck- or CD4-expressing T-lineage cells. Mice initially showed modest thymic hyperplasia and subsequently developed expanding and infiltrating T-cell lymphomas, leading to a premature death within 5 to 23 weeks. Frequently, all thymocyte and peripheral T-cell populations displayed phenotypes characteristic for immature developing thymocyte precursors and shared elevated levels of clonally rearranged T-cell receptor (TCR) beta chains. In concert, CD2, CD5, CD3epsilon and CD44, proteins associated with increased expression and signaling capacity of both the immature pre-TCR and the mature alphabetaTCR, were more abundantly expressed, reflecting a constitutive state of activation. Although most T-cell lymphomas had acquired the capability to infiltrate the periphery, not all populations left the thymus and expanded clonally exclusively in the thymus. In line with this, only transplantation of thymocytes with infiltrating capacity gave rise to T-cell lymphoma in immunodeficient recipients. These results indicate that T-cell-specific Pten deletion during various stages of thymocyte development gives rise to clonally expanding T-cell lymphomas that frequently infiltrate the periphery, but originate in the thymus.

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

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

Key factors in the organized chaos of early T cell development

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

Importance of the CD3γ Ectodomain Terminal β-Strand and Membrane Proximal Stalk in Thymic Development and Receptor Assembly

CD3epsilongamma and CD3epsilondelta are noncovalent heterodimers; each consists of Ig-like extracellular domains associated side-to-side via paired terminal beta-strands that are linked to individual subunit membrane proximal stalk segments. CD3epsilon, CD3gamma, and CD3delta stalks contain the RxCxxCxE motif. To investigate the functional importance of a CD3 stalk and terminal beta-strand, we created a CD3gamma double mutant CD3gamma(C82S/C85S) and a CD3gamma beta-strand triple mutant CD3gamma(Q76S/Y78A/Y79A) for use in retroviral transduction of lymphoid progenitors for comparison with CD3gammawt. Although both mutant CD3gamma molecules reduced association with CD3epsilon in CD3epsilongamma heterodimers, CD3gamma(Q76S/Y78A/Y79A) abrogated surface TCR expression whereas CD3gamma(C82S/C85S) did not. Furthermore, CD3gamma(C82S/C85S) rescued thymic development in CD3gamma(-/-) fetal thymic organ culture. However, the numbers of double-positive and single-positive thymocytes after CD3gamma(C82S/C85S) transduction were significantly reduced despite surface pre-TCR and TCR expression comparable to that of CD3gamma(-/-) thymocytes transduced in fetal thymic organ culture with a retrovirus harboring CD3gammawt cDNA. Furthermore, double-negative thymocyte development was perturbed with attenuated double-negative 3/double-negative 4 maturation and altered surface-expressed CD3epsilongamma, as evidenced by the loss of reactivity with CD3gamma N terminus-specific antisera. Single histidine substitution of either CD3gamma stalk cysteine failed to restore CD3epsilongamma association and conformation in transient COS-7 cell transfection studies. Thus, CD3gamma(C82) and CD3gamma(C85) residues likely are either reduced or form a tight intrachain disulfide loop rather than contribute to a metal coordination site in conjunction with CD3epsilon(C80) and CD3epsilon(C83). The implications of these results for CD3epsilongamma and TCR structure and signaling function are discussed.

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

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

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

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

Strength of signal: a fundamental mechanism for cell fate specification

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

T-cell development, doing it in a dish

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

Requirement of the Fc receptor common gamma-chain for gamma delta T cell-mediated promotion of murine experimental autoimmune encephalomyelitis

Immunoglobulin Fcgamma receptors (FcgammaR) are comprised of a ligand-binding alpha-chain that sometimes associates with a cell signaling common gamma-chain. These receptors comprise an important family of effector molecules that link humoral and cell-mediated adaptive immunity and regulate innate immunity. Recent animal studies suggest that FcgammaR in general, and FcR alpha-chains in particular, are required for full development of experimental autoimmune encephalomyelitis (EAE). We show here that deletion of the gamma-chain renders mice resistant to EAE, whereas deletion of the alpha-chains of FcgammaRI, FcgammaRIIB and FcgammaRIII has no protective effect. Susceptibility to EAE is fully restored in common gamma-chain-/- mice into which wild-type splenocytes are adoptively transferred, but EAE is not restored in common gamma-chain-/- mice given wild-type splenocytes depleted of gammadelta T cells. These data indicate that although the common gamma-chain is required for full development of EAE in mice, this requirement is likely FcgammaR alpha-chain-independent. Expression of the common gamma-chain by gammadelta T cells, probably in conjunction with the T cell receptor/CD3 complex, is likely the key requirement for full development of EAE.

The TCR C beta FG loop regulates alpha beta T cell development

The TCRbeta chain constant domain contains an unusually elongated, solvent-exposed FG loop. This structural element forms one component of an alphabeta TCR cavity against which CD3epsilongamma may abut to facilitate Ag-specific signaling. Consistent with this notion, in the present study we show that N15alphabeta TCR transfectants expressing a FG loop-deleted chain (betaDeltaFG) stimulate less tyrosine protein phosphorylation than those bearing a wild-type beta-chain (betawt) upon TCR cross-linking. Furthermore, coimmunoprecipitation studies suggest a weakened association between the CD3epsilongamma heterodimer and the beta-chain in TCR complexes containing the betaDeltaFG variant. To further investigate the biologic role of the Cbeta FG loop in development, we competitively reconstituted the thymus of Ly5 congenic or RAG-2-/- mice using bone marrow cells from betawt or betaDeltaFG transgenic C57BL/6 (B6) mice. Both betawt and betaDeltaFG precursor cells generate thymocytes representative of all maturational stages. However, betaDeltaFG-expressing thymocytes dominate during subsequent development, resulting in an excess of betaDeltaFG-expressing peripheral T cells with reduced proliferative and cytokine production abilities upon TCR stimulation. Collectively, our results show that the unique Cbeta FG loop appendage primarily controls alphabeta T cell development through selection processes.

Mechanistic basis of pre–T cell receptor–mediated autonomous signaling critical for thymocyte development

The pre-T cell receptor (TCR) is crucial for early T cell development and is proposed to function in a ligand-independent way. However, the molecular mechanism underlying the autonomous signals remains elusive. Here we show that the pre-TCR complex spontaneously formed oligomers. Specific charged residues in the extracellular domain of the pre-TCR alpha-chain mediated formation of the oligomers in vitro. Alteration of these residues eliminated the ability of the pre-TCR alpha-chain to support pre-TCR signaling in vivo. Dimerization but not raft localization of CD3epsilon was sufficient to simulate pre-TCR function and promote beta-selection. These results suggest that the pre-TCR complex can deliver its signal autonomously through oligomerization of the pre-TCR alpha-chain mediated by charged residues.

Influence of Bacillus Calmette-Guèrin vaccination at birth and 2 months old age on the peripheral blood T-cell subpopulations [gamma/delta and alpha-beta T cell]

The neonatal immune system is immature and may be affected by Bacillus Calmette-Guèrin (BCG) vaccine. We investigated the influence of BCG given at two different ages on the peripheral blood (PB) T-cell subpopulations. Forty full term healthy newborns were randomly chosen. Twenty of them were vaccinated with BCG at birth (group I) and the remaining at the age of 2 months (group II). The cell analysis were carried out before (pre-BCGI and pre-BCGII), and 2 months after (post-BCGI and post-BCGII) the vaccination. The analysis of the gamma/delta and alpha/beta T-cell receptor (TCR) antigens was done by two-colour flowcytometer. The purified protein derivative (PPD) response was investigated 2 months after vaccination. The results showed that although T-cell (TCR+ cell) counts showed no difference in PB before and after vaccination in both study groups, the total lymphocyte and non-T cell (TCR- cell) populations increased significantly whereas alphabetaT-cell population significantly decreased after vaccination. On the contrary, gammadeltaT-cell counts in PB increased significantly 2 months after vaccination in group I but not in group II. Total lymphocyte and non-T cell counts in vaccinated infants at 2 months of age (post-BCGI) were significantly higher than in unvaccinated infants of the same age whereas alphabetaT-cell count in vaccinated infants was significantly low. However, total T-cell and gammadeltaT-cell counts showed no difference. PPD positivity was similar in both study groups (61% in group I, 66% in group II). Neither alphabetaT- nor gammadeltaT-cell counts were different in PPD positive and PPD negative infants. Our study shows that BCG causes marked quantitative changes in the PB T-cell subpopulations in young infants.

Conditional deletion reveals a cell-autonomous requirement of SLP-76 for thymocyte selection

The SH2 domain containing leukocyte phosphoprotein of 76 kD (SLP-76) is critical for pre-TCR-mediated maturation to the CD4+CD8+ double positive (DP) stage in the thymus. The absolute block in SLP-76null mice at the CD4-CD8-CD44-CD25+ (double-negative 3, DN3) stage has hindered our understanding of the role of this adaptor in alphabeta TCR-mediated signal transduction in primary thymocytes and peripheral T lymphocytes. To evaluate the requirements for SLP-76 in these events, we used a cre-loxP approach to generate mice that conditionally delete SLP-76 after the DN3 checkpoint. These mice develop DP thymocytes that express the alphabeta TCR on the surface, but lack SLP-76 at the genomic DNA and protein levels. The DP compartment has reduced cellularity in young mice and fails to undergo positive selection to CD4+ or CD8+ single positive (SP) cells in vivo or activation-induced cell death in vitro. A small number of CD4+SP thymocytes are generated, but these cells fail to flux calcium in response to an alphabeta TCR-generated signal. Peripheral T cells are reduced in number, lack SLP-76 protein, and have an abnormal surface phenotype. These studies show for the first time that SLP-76 is required for signal transduction through the mature alphabeta TCR in primary cells of the T lineage.

Gammadelta T cell development--having the strength to get there

Gammadelta T cells play critical roles in immune regulation, tumour surveillance and specific primary immune responses. Mature gammadelta cells derive from thymic precursors that also generate alphabeta T cells. Recent reports have highlighted the impact of the strength of signal received via the T cell receptor on T cell lineage commitment, and the importance of cross-talk between committed alphabeta thymocytes and bipotential progenitors for normal gammadelta T cell differentiation. Studies on T cell receptor-mediated selection of gammadelta cells have supported the view that these unconventional T cells are positively rather than negatively selected on cognate self antigen.

Attenuation of gammadeltaTCR signaling efficiently diverts thymocytes to the alphabeta lineage

The role of the T cell antigen receptor complex (TCR) in alphabeta/gammadelta lineage commitment remains controversial, in particular whether different TCR isoforms intrinsically favor adoption of a certain lineage. Here, we demonstrate that impairing the signaling capacity of a gammadeltaTCR complex enables it to efficiently direct thymocytes to the alphabeta lineage. In the presence of a ligand, a transgenic gammadeltaTCR mediates almost exclusive adoption of the gammadelta lineage, while in the absence of ligand, the same gammadeltaTCR promotes alphabeta lineage development with efficiency comparable to the pre-TCR. Importantly, attenuating gammadeltaTCR signaling through Lck deficiency causes reduced ERK1/2 activation and Egr expression and diverts thymocytes to the alphabeta lineage even in the presence of ligand. Conversely, ectopic Egr overexpression favors gammadelta T cell development. Our data support a model whereby gammadelta versus alphabeta lineage commitment is controlled by TCR signal strength, which depends critically on the ERK MAPK-Egr pathway.

Gene expression characteristics of CD28null memory phenotype CD8+ T cells and its implication in T-cell aging

Accumulation of CD28(null)CD8(+) T cells is considered as one of the hallmarks of aging in the human immune system. However, the precise changes of CD28(null)CD8(+) T cells, compared to those of the precursor CD28(+)CD8(+) memory T cells, have not been determined. In this study, we present an analysis of the global gene expression profiles of CD28(+) and CD28(null) memory phenotype CD8(+) T cells. These two CD8(+) T subsets exhibited an overall similar gene expression profile with only a few dozen genes that were differentially expressed. A wide range of functions, including co-stimulation, effector activity, signaling, and transcription, were possessed by these differentially expressed genes, reflecting significant functional changes of CD28(null) memory phenotype CD8(+) T cells from their CD28(+) counterparts. In addition, CD28(null) memory CD8(+) T cells expressed several natural killer cell receptors and high levels of granzymes, perforin, and FasL, indicating an increasing capacity for cytotoxicity during memory CD8(+) T-cell aging. Interestingly, in vitro culture of these two subsets with interleukin-15 showed that similar gene expression changes occurred in both subsets. Our analysis provides the gene expression portraits of CD28(null) memory phenotype CD8(+) T cells and alteration from their CD28(+) counterparts and suggests potential mechanisms of T-cell aging.

Monoclonal antibodies that identify the CD3 molecules expressed specifically at the surface of porcine gammadelta-T cells

The CD3 antigen is a surface structure associated with the T-cell receptor (TCR) to form a complex involved in antigen recognition and signal transduction. Reports on the structures of the CD3 molecules associated with alphabeta- and gammadelta-TCR have been contradictory. To investigate this issue, we raised a panel of monoclonal antibodies (mAb) against purified porcine CD3 molecules. Unlike the conventional anti-CD3, these mAb reacted specifically with peripheral gammadelta-T cells, but not with alphabeta-T cells. Immunoprecipitation showed that the antibody recognized a subset of CD3 molecules that were associated with gammadelta-TCR. Also unlike the conventional anti-CD3, these mAb, though directed at two different epitope groups, failed to induce antigenic modulation, T-cell proliferation and CD3-redirected cytotoxicity. Taken together, these results suggest that there are differences in the antigenicity, signal transduction potentials and probably structural differences between the CD3 molecules expressed at the surface of alphabeta- and gammadelta-T cells.

TCR Signal Strength Influences αβ/γδ Lineage Fate

Signals transduced by T cell antigen receptors (TCRs) have been shown to be critical for alphabeta and gammadelta T cell development, but their role in lineage determination remains poorly defined. Two models have been forwarded for alphabeta/gammadelta lineage choice: the instructive model and the stochastic model. Recent data, however, are inconsistent with either model. In this study, we devised an experimental system in which lineage fate was controlled exclusively by the gammadeltaTCR. We then analyzed the impact of TCR signal strength on alphabeta/gammadelta lineage development by altering the surface expression or signaling potential of the gammadeltaTCR complex. We found that increasing the gammadeltaTCR signal strength favored gammadelta lineage development, whereas weakening the gammadeltaTCR signal favored alphabeta lineage development. These results support a model in which the strength of the TCR signal is a critical determinant in the lineage fate decision.