Positive selection of dendritic epidermal gammadelta T cell precursors in the fetal thymus determines expression of skin-homing receptors

Tec family kinases: Itk signaling and the development of NKT αβ and γδ T cells

The Tec family tyrosine kinase interleukin-2 inducible T-cell kinase (Itk) is predominantly expressed in T cells and has been shown to be critical for the development, function and differentiation of conventional αβ T cells. However, less is known about its role in nonconventional T cells such as NKT and γδ T cells. In this minireview, we discuss evidence for a role for Itk in the development of invariant NKT αβ cells, as well as a smaller population NKT-like γδ T cells. We discuss how these cells take what could be the same signaling pathway regulated by Itk, and interpret it to give different outcomes with regards to development and function.

γδ T cell receptor ligands and modes of antigen recognition

T lymphocytes expressing the γδ-type of T cell receptors (TCRs) for antigens contribute to all aspects of immune responses, including defenses against viruses, bacteria, parasites and tumors, allergy and autoimmunity. Multiple subsets have been individualized in humans as well as in mice and they appear to recognize in a TCR-dependent manner antigens as diverse as small non-peptidic molecules, soluble or membrane-anchored polypeptides and molecules related to MHC antigens on cell surfaces, implying diverse modes of antigen recognition. We review here the γδ TCR ligands which have been identified along the years and their characteristics, with emphasis on a few systems which have been extensively studied such as human γδ T cells responding to phosphoantigens or murine γδ T cells activated by allogeneic MHC antigens. We discuss a speculative model of antigen recognition involving simultaneous TCR recognition of MHC-like and non-MHC ligands which could fit with most available data and shares many similarities with the classical model of MHC-restricted antigen recognition for peptides or lipids by T cells subsets with αβ-type TCRs.

Cutting edge: Intrinsic programming of thymic γδT cells for specific peripheral tissue localization

Various innate-like T cell subsets preferentially reside in specific epithelial tissues as the first line of defense. However, mechanisms regulating their tissue-specific development are poorly understood. Using the prototypical skin intraepithelial γδT cells (sIELs) as a model, we show in this study that a TCR-mediated selection plays an important role in promoting acquisition of a specific skin-homing property by fetal thymic sIEL precursors for their epidermal location, and the skin-homing potential is intrinsically programmed even before the selection. In addition, once localized in the skin, the sIEL precursors develop into sIELs without the requirement of further TCR-ligand interaction. These studies reveal that development of the tissue-specific lymphocytes is a hard-wired process that targets them to specific tissues for proper functions.

CCR10 is important for the development of skin-specific gammadeltaT cells by regulating their migration and location

Unlike conventional αβ T cells, which preferentially reside in secondary lymphoid organs for adaptive immune responses, various subsets of unconventional T cells, such as the γδ T cells with innate properties, preferentially reside in epithelial tissues as the first line of defense. However, mechanisms underlying their tissue-specific development are not well understood. We report in this paper that among different thymic T cell subsets fetal thymic precursors of the prototypic skin intraepithelial Vγ3(+) T lymphocytes (sIELs) were selected to display a unique pattern of homing molecules, including a high level of CCR10 expression that was important for their development into sIELs. In fetal CCR10-knockout mice, the Vγ3(+) sIEL precursors developed normally in the thymus but were defective in migrating into the skin. Although the earlier defect in skin-seeding by sIEL precursors was partially compensated for by their normal expansion in the skin of adult CCR10-knockout mice, the Vγ3(+) sIELs displayed abnormal morphology and increasingly accumulated in the dermal region of the skin. These findings provide definite evidence that CCR10 is important in sIEL development by regulating the migration of sIEL precursors and their maintenance in proper regions of the skin and support the notion that unique homing properties of different thymic T cell subsets play an important role in their peripheral location.

γδ T cells enhance autoimmunity by restraining regulatory T cell responses via an interleukin-23-dependent mechanism

Mice that lack interleukin-23 (IL-23) are resistant to T cell-mediated autoimmunity. Although IL-23 is a maturation factor for T helper 17 (Th17) cells, a subset of γδ T cells expresses the IL-23 receptor (IL-23R) constitutively. Using IL-23R reporter mice, we showed that γδ T cells were the first cells to respond to IL-23 during experimental autoimmune encephalomyelitis (EAE). Although γδ T cells produced Th17 cell-associated cytokines in response to IL-23, their major function was to prevent the development of regulatory T (Treg) cell responses. IL-23-activated γδ T cells rendered αβ effector T cells refractory to the suppressive activity of Treg cells and also prevented the conversion of conventional T cells into Foxp3(+) Treg cells in vivo. Thus, IL-23, which by itself has no direct effect on Treg cells, is able to disarm Treg cell responses and promote antigen-specific effector T cell responses via activating γδ T cells.

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

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

gammadeltaTCR ligands and lineage commitment

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

Epidermal T cells and wound healing

The murine epidermis contains resident T cells that express a canonical gammadelta TCR. These cells arise from fetal thymic precursors and use a TCR that is restricted to the skin in adult animals. These cells assume a dendritic morphology in normal skin and constitutively produce low levels of cytokines that contribute to epidermal homeostasis. When skin is wounded, an unknown Ag is expressed on damaged keratinocytes. Neighboring gammadelta T cells then round up and contribute to wound healing by local production of epithelial growth factors and inflammatory cytokines. In the absence of skin gammadelta T cells, wound healing is impaired. Similarly, epidermal T cells from patients with healing wounds are activated and secreting growth factors. Patients with nonhealing wounds have a defective epidermal T cell response. Information gained on the role of epidermal-resident T cells in the mouse may provide information for development of new therapeutic approaches to wound healing.

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

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

Differential roles of IL-2-inducible T cell kinase-mediated TCR signals in tissue-specific localization and maintenance of skin intraepithelial T cells

Tissue-specific innate-like gammadelta T cells are important components of the immune system critical for the first line of defense, but mechanisms underlying their tissue-specific development are poorly understood. Our study with prototypical skin-specific intraepithelial gammadeltaT lymphocytes (sIELs) found that among different thymic gammadelta T cell subsets fetal thymic precursors of sIELs specifically acquire a unique skin-homing property after positive selection, suggesting an important role of the TCR selection signaling in "programming" them for tissue-specific development. In this study, we identified IL-2-inducible T cell kinase (ITK) as a critical signal molecule regulating the acquirement of the skin-homing property by the fetal thymic sIEL precursors. In ITK knockout mice, the sIEL precursors could not undergo positive selection-associated upregulation of thymus-exiting and skin-homing molecules sphingosine-1-phosphate receptor 1 and CCR10 and accumulated in the thymus. However, the survival and expansion of sIELs in the skin did not require ITK-transduced TCR signaling, whereas its persistent activation impaired sIEL development by inducing apoptosis. These findings provide insights into molecular mechanisms underlying differential requirements of TCR signaling in peripheral localization and maintenance of the tissue-specific T cells.

gammadelta T cell subsets: a link between TCR and function?

The gammadelta T lmphocytes are often divided into subsets based upon expression of certain TCR components. This division was initially made because gammadelta T cells residing in particular epithelia were found to show tissue specific differences in their TCRs. Many examples now show that gammadelta T cell subsets also appear to be biased to carry out particular functions. This suggests that particular gammadelta TCR types direct the cells to acquire a certain type of functional programming during thymic development. Here, we describe functionally distinct, TCR-defined gammadelta T cell subsets, and evidence that their functions are predetermined in the thymus.

Krüppel-like factor 2 regulates trafficking and homeostasis of gammadelta T cells

gammadelta T cells are generated in the thymus and traffic to secondary lymphoid organs and epithelial surfaces, where they regulate immune responses. alphabeta T cells require sphingosine 1-phosphate receptor type 1 (S1P(1)) and CD62L for thymic emigration and circulation through secondary lymphoid organs. Both of these genes are regulated by the transcription factor Krüppel-like factor 2 (KLF2) in conventional alphabeta T cells. It is unclear if gammadelta T cells use similar mechanisms. In this study, we show that thymic gammadelta T cells express S1P(1) and that it is regulated by KLF2. Furthermore, KLF2 and S1P(1)-deficient gammadelta T cells accumulate in the thymus and fail to populate the secondary lymphoid organs or gut, in contrast to the expectation from published work. Interestingly, KLF2 but not S1P(1) deficiency led to the expansion of a usually rare population of CD4(+) promyelocytic leukemia zinc finger(+) "gammadelta NKT" cells. Thus, KLF2 is critically important for the homeostasis and trafficking of gammadelta T cells.

Ligand recognition during thymic development and gammadelta T cell function specification

gammadelta T cells develop in the thymus before entering the periphery. Recent work suggests that thymic development does little to constrain gammadelta T cell antigen specificities, but instead determines their effector fate. When triggered through the T cell receptor, ligand-naïve gammadelta T cells produce IL-17, ligand-experienced cells make IFN-gamma and those that are strongly self-reactive make IL-4. Importantly, gammadelta T cells are able to make cytokines immediately upon TCR engagement. These characteristics allow gammadelta T cells to initiate an acute inflammatory response to pathogens and to host antigens revealed by injury. These advances warrant a fresh look at how gammadelta T cells may function in the immune system.

Embryonic trafficking of gammadelta T cells to skin is dependent on E/P selectin ligands and CCR4

Dendritic epidermal T cells (DETC) express an invariant Vgamma3/Vdelta1 T-cell receptor, appear in fetal epidermis, and form a population of resident epidermal T cells. Their temporal development in the thymus has been studied extensively. However, little is known about the mechanisms involved in the embryonic trafficking of DETC from thymus to epidermis. We demonstrate that DETC in adult skin, as well as the DETC precursors in fetal thymus, express E and P selectin ligands (E- and P-lig). Mice deficient in alpha1,3 fucosyltransferases IV and VII (FTIV/VII) cannot synthesize the carbohydrate motifs that form key elements of these selectin ligands. The numbers of DETC in the epidermis of FTIV/VII(-/-) mice were dramatically reduced compared with normal mice. However, the development of DETC precursors in fetal thymus was identical in normal and FTIV/VII(-/-) mice, suggesting that the DETC precursors produced in FTIV/VII(-/-) mice could not traffic effectively to skin because they lack E- and P-lig. We tested this hypothesis by daily injection of blocking antibodies against E and P selectin into pregnant mice. Mice born from dams treated with anti-selectin antibodies, but not those born from dams treated with isotype control, had significantly diminished numbers of DETC. To test the role of chemokine receptors in DETC skin homing, we examined skin from CCR4(-/-) and CCR10(-/-) mice, respectively. DETC were significantly reduced in CCR4(-/-) mice but were present at normal levels in CCR10(-/-) mice. Our results present evidence for the crucial role of trafficking molecules in embryonic migration of DETC precursors to skin.

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

BACKGROUND

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

METHODOLOGY/PRINCIPAL FINDINGS

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

CONCLUSIONS

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

The innate immune response to uropathogenic Escherichia coli involves IL-17A in a murine model of urinary tract infection

Uropathogenic Escherichia coli is the causative agent for >80% of uncomplicated urinary tract infections (UTIs). Uropathogenic E. coli strains express a number of virulence and fitness factors that allow successful colonization of the mammalian bladder. To combat this, the host has distinct mechanisms to prevent adherence to the bladder wall and to detect and kill uropathogenic E. coli in the event of colonization. In this study, we investigated the role of IL-17A, an innate-adaptive immunomodulatory cytokine, during UTI using a murine model. Splenocytes isolated from mice infected by the transurethral route robustly expressed IL-17A in response to in vitro stimulation with uropathogenic E. coli Ags. Transcript expression of IL-17A in the bladders of infected mice correlated with a role in the innate immune response to UTI, and gammadelta cells seem to be a key source of IL-17A production. Although IL-17A seems to be dispensable for the generation of a protective response to uropathogenic E. coli, its importance in innate immunity is demonstrated by a defect in acute clearance of uropathogenic E. coli in IL-17A(-/-) mice. This clearance defect is likely a result of deficient cytokine and chemokine transcripts and impaired macrophage and neutrophil influx during infection. These results show that IL-17A is a key mediator for the innate immune response to UTIs.

Recombination signal sequence-associated restriction on TCRdelta gene rearrangement affects the development of tissue-specific gammadelta T cells

Assembly of TCRalpha and TCRdelta genes from the TCRalpha/delta locus is tightly controlled for the proper generation of alphabeta and gammadelta T cells. Of >100 shared variable gene segments in the TCRalpha/delta locus, only a few are predominantly used for the TCRdelta gene assembly, while most are for TCRalpha. However, the importance and mechanisms of the selective variable gene rearrangement for T cell development are not fully understood. We report herein that the development of a tissue-specific gammadelta T cell population is critically affected by recombination signal sequence-associated restriction on the variable gene usage for TCRdelta assembly. We found that the development of substitute skin gammadelta T cells in mice deficient of the TCRgamma3 gene, which is used in wild-type skin gammadelta T cells, was drastically affected by the strain background. A Vgamma2(+) skin gammadelta T cell population developed in mice of the B6 but not the 129 strain backgrounds, due to a difference in the rearrangement of endogenous Vdelta7(+) TCRdelta genes, which paired with the Vgamma2(+) TCRgamma gene to generate the Vgamma2/Vdelta7(+) skin gammadelta T cell precursors in fetal thymi of the B6 background mice. The defective TCRdelta rearrangement of the 129-"Vdelta7" gene was associated with specific variations in its recombination signal sequence, which renders it poorly compatible for rearrangement to Ddelta genes. These findings provide the first direct evidence that recombination signal sequence-associated restriction on the variable gene usage for TCRalpha/delta gene assembly plays an important role in T cell development.

Cutting edge: Gammadelta intraepithelial lymphocytes of the small intestine are not biased toward thymic antigens

gammadelta Tau cells, together with alphabeta Tau cells, are abundantly present in the epithelial layer of the small intestine (IEL) and are essential for the host's first line of defense. Whether or not gammadelta IELs, like alphabeta IELs, are derived from thymocytes that encounter self-Ags in the thymus is unclear. In this study, we report that a natural population of gammadelta T cells that are specific for the nonclassical MHC class I molecules T10 and T22 are present in the IEL compartment of mice that do not express T10/T22. Furthermore, the small intestinal homing receptor CCR9 is preferentially expressed on gammadelta thymocytes that have yet to encounter a ligand, and gammadelta thymocytes with high affinity for self-ligand are CCR9(low). These observations suggest that the Ag-specific repertoire of gammadelta IELs is not biased toward thymic Ags. Instead, gammadelta IELs appear suited to respond to novel Ags revealed in pathological settings.

Id3 restricts the developmental potential of gamma delta lineage during thymopoiesis

Most T cell progenitors develop into the alphabeta T cell lineage with the exception of a small fraction contributing to the gammadelta lineage throughout postnatal life. T cell progenitors usually commit to the alphabeta lineage upon the expression of a fully rearranged and functional TCRbeta gene, and most cells that fail to produce a functional TCRbeta-chain will die instead of adopting the alternative gammadelta T cell fate. What prevents these cells from continuing TCRgamma rearrangement and adopting the gammadelta T cell fate is not known. In this study, we show that functional loss of Id3 results in a significant increase of gammadelta T cell production from progenitor cells undergoing TCRbeta rearrangement. The enhanced gammadelta T cell development correlated with increased TCRgamma gene rearrangement involving primarily Vgamma1.1 in Id3 deficient mice. We further show that Id3 deficiency promotes gammadelta T cell production in a manner independent of TCRbeta-chain expression. Our data indicates that Id3 suppresses Vgamma1.1 rearrangement and gammadelta lineage potential among T cell progenitors that have completed TCRbeta gene rearrangement without producing a functional TCRbeta protein.

Costimulation of dendritic epidermal gammadelta T cells by a new NKG2D ligand expressed specifically in the skin

Dendritic epidermal T cells (DETCs) are a highly specialized population of gammadelta T cells that resides in the murine skin and participates in wound healing and tumor surveillance. Despite the expression of other stimulatory receptors on these cells, mechanisms involving activation have focused primarily on the invariant Vgamma3-Vdelta1 TCR expressed by DETCs. All DETCs also express the activating NKG2D receptor, but the role of NKG2D in DETC activation remains unclear, as does the identity of NKG2D ligands that are functionally expressed in the skin. In this study, we document the cloning of an NKG2D ligand H60c that is expressed specifically in the skin and in cultured keratinocytes and demonstrate its role in the activation of DETCs and NK cells. The ligand is unique among NKG2D ligands in being up-regulated in cultured keratinocytes, and its interaction with NKG2D is essential for DETC activation. Importantly, it is shown that engagement of NKG2D is not sufficient to activate DETCs, but instead provides a costimulatory signal that is nevertheless essential for activating DETCs in response to stimulation with keratinocytes.

Thymic maturation determines gammadelta T cell function, but not their antigen specificities

gammadelta T cells contribute uniquely to host immune defense, but how they do so remains unclear. Recent work suggests that thymic selection does little to constrain gammadelta T cell antigen specificities, but instead determines their effector fate. When triggered through the T cell receptor, ligand-experienced cells make IFNgamma, whereas ligand-naïve gammadelta T cells produce IL-17, a major initiator of inflammation. These advances warrant a fresh look at how gammadelta T cells may function in the immune system.

gammadelta T cells develop independently of Aire

Mutations in the transcriptional regulator Aire disrupt thymic alphabeta T cell selection, causing in humans Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED). However, it is not known whether Aire is needed for normal gammadelta T cell development. We show that Aire(-/-) mice have a normal frequency of gammadelta T cells, with TCR repertoire comparable to that of wild-type mice, and normal amount of TCR Cdelta mRNA in ileum and skin. gammadelta T cells did not express increased amounts of CD25 or display hyperproliferation, and were not involved in pathological salivary gland infiltrates. Lastly, the frequency of circulating gammadelta T cells was similar in APECED patients and healthy controls. These data indicate that gammadelta T cells develop independently of Aire and are unlikely to have a significant pathogenetic or protective role in APECED. The antigens responsible for gammadelta and alphabeta T cell selection are thus probably largely different.

gammadelta T cell selection: is anyone useless?

In this issue of Immunity, a study by Jensen et al. (2008) suggests that T cell-receptor engagement during development affects gammadelta T cell polarization toward either interferon-gamma or interleukin-17 production. This might underlie their unique innate ability to regulate inflammation.

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.

Gene placement and competition control T cell receptor gamma variable region gene rearrangement

The production of distinct sets of T cell receptor (TCR) γδ⁺ T cells occurs in an ordered fashion in thymic development. The Vγ3 and Vγ4 genes, located downstream in the TCRγ Cγ1 gene cluster, are expressed by the earliest waves of developing TCRγδ⁺ T cells in the fetal thymus, destined for intraepithelial locations. Upstream Vγ2 and Vγ5 genes are expressed in later waves in the adult and constitute most TCRγδ⁺ T cells in secondary lymphoid tissue. This developmental pattern is caused in part by a preference for rearrangements of the downstream Vγ3 and Vγ4 genes in the early fetal stage, which switches to a preference for rearrangements of the upstream Vγ2 and Vγ5 gene rearrangements in the adult. Our gene targeting studies show that the downstream Vγ genes rearrange preferentially in the early fetal thymus because of their downstream location, independent of promoter or recombination signal sequences and unrelated to the extent of germline transcription. Remarkably, gene deletion studies show that the downstream Vγ genes competitively inhibit upstream Vγ rearrangements at the fetal stage. These data provide a mechanism for specialization of the fetal thymus for the production of T cells expressing specific Vγ genes.

Evidence for the involvement of lung-specific gammadelta T cell subsets in local responses to Streptococcus pneumoniae infection

Although γδ T cells are involved in the response to many pathogens, the dynamics and heterogeneity of the local γδ T cell response remains poorly defined. We recently identified γδ T cells as regulators of macrophages and dendritic cells during the resolution of Streptococcus pneumoniae-mediated lung inflammation. Here, using PCR, spectratype analysis and flow cytometry, we show that multiple γδ T cell subsets, including those bearing Vγ1, Vγ4 and Vγ6 TCR, increase in number in the lungs of infected mice, but not in associated lymphoid tissue. These γδ T cells displayed signs of activation, as defined by CD69 and CD25 expression. In vivo BrdU incorporation suggested that local expansion, rather than recruitment, was the principal mechanism underlying this increase in γδ T cells. This conclusion was supported by the finding that pulmonary γδ T cells, but not αβ T cells, isolated from mice that had resolved infection exhibited lung-homing capacity in both naive and infected recipients. Together, these data provide novel insights into the origins of the heterogeneous γδ T cell response that accompanies lung infection, and the first evidence that inflammation-associated γδ T cells may exhibit distinct tissue-homing potential.

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

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

Germ-line and rearranged Tcrd transcription distinguish bona fide NK cells and NK-like gammadelta T cells

NK cells and gammadelta T cells are distinct subsets of lymphocytes that contextually share multiple phenotypic and functional characteristics. However, the acquisition and the extent of these similarities remain poorly understood. Here, using T cell receptor delta locus-histone 2B-enhanced GFP (Tcrd-H2BEGFP) reporter mice, we show that germ-line transcription of Tcrd occurs in all maturing NK cells. We also describe a population of mouse NK-like cells that are indistinguishable from "bona fide" NK cells using standard protocols. Requirements for V(D)J recombination and a functional thymus, along with very low-level expression of surface TCRgammadelta but high intracellular CD3, define these cells as gammadelta T cells. "NK-like gammadelta T cells" are CD127+, have a memory-activated phenotype, express multiple NK cell receptors and readily produce interferon-gamma in response to IL-12/IL-18 stimulation. The close phenotypic resemblance between NK cells and NK-like gammadelta T cells is a source of experimental ambiguity in studies bridging NK and T cell biology, such as those on thymic NK cell development. Instead, it ascribes chronic TCRgammadelta engagement as a means of acquiring NK-like function.

Key factors in the organized chaos of early T cell development

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

Development and selection of gammadelta T cells

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

gammadelta T-cell receptors: functional correlations

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

Stage-dependent reactivity of thymocytes to self-peptide--MHC complexes

In mice that express a transgene for the 2C T cell antigen-receptor (TCR) and lack a recombinase-activating gene (2C(+)RAG(-/-) mice) most of the peripheral T cells are CD8(+), a few are CD4(+), and a significant fraction are CD4(-)CD8(-) [double negative (DN)]. The DN 2C cells, like DN T cells that are abundant in various other alphabeta TCR-transgenic mice, appear to be derived directly from DN thymocytes that prematurely express the TCR transgene. The DN 2C cells are virtually absent in mice deficient in major histocompatibility complex class II (MHC-II) but more abundant in mice deficient in MHC-I, suggesting that the DN 2C thymocytes are positively selected by self-peptide-MHC-II (pMHC-II) complexes and negatively selected by self-pMHC-I complexes. The pMHC-I complexes, however, positively select CD8(+) 2C T cells in the same mice. The different effects of thymic pMHC-I on DN and CD8(+) thymocytes are consistent with the finding that DN 2C thymocytes are more sensitive than more mature CD4(+)CD8(+) [double positive (DP)] thymocytes to a weak pMHC-I agonist for the 2C TCR. Together with previous evidence that DP thymocytes respond more sensitively than T cells in the periphery to weak pMHC agonists, the findings suggest progressive decreases in responsiveness to self-pMHC-I complexes as thymocytes develop from DN to DP thymocytes and then to mature naïve T cells in the periphery.

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.

The unconventional role of LT alpha beta in T cell differentiation

Lymphotoxin (LT)alphabeta, a member of the tumor necrosis factor cytokine superfamily, and its receptor, the LTbeta receptor (LTbetaR), have a well defined role in secondary lymphoid organogenesis but an unexpected function in T cell differentiation. Although earlier studies indicated that conventional T cell subsets were normal in mice deficient in the LTbetaR pathway, accumulating evidence indicates that the LTalphabeta-LTbetaR pathway has a pivotal role in the ontogeny of unconventional T cells, including gammadelta T cells and invariant natural killer T cells. The LTbetaR pathway seems to operate at distinct levels during thymic development. Double positive thymocytes regulate the differentiation of early thymocyte progenitors and gammadelta T cells through a mechanism dependent on LTbetaR. In addition, LTbetaR signaling in thymic stroma was proposed to affect central tolerance to peripherally restricted antigens. These findings highlight the complex cellular crosstalk between lymphoid and stromal compartments during thymic differentiation.

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.

Antigen recognition by γδ T cells

gammadelta T cells contribute to host immune competence uniquely. This is most likely because they have distinctive antigen-recognition properties. While the basic organization of gammadelta T-cell receptor (TCR) loci is similar to that of alphabeta TCR loci, there is a striking difference in how the diversity of gammadelta TCRs is generated. gammadelta and alphabeta T cells have different antigen-recognition requirements and almost certainly recognize a different set of antigens. While it is unclear what most gammadelta T cells recognize, the non-classical major histocompatibility complex class I molecules T10 and T22 were found to be the natural ligands for a sizable population (0.2-2%) of murine gammadelta T cells. The recognition of T10/T22 may be a way by which gammadelta T cells regulate cells of the immune system, and this system has been used to determine the antigen-recognition determinants of gammadelta T cells. T10/T22-specific gammadelta T cells have TCRs that are diverse in both V gene usage and CDR3 sequences. Their Vgamma usage reflects their tissue origin, and their antigen specificity is conferred by a motif in the TCR delta chain that is encoded by V and D segments and by P-nucleotide addition. Sequence variations around this motif modulate affinities between TCRs and T10/T22. That this CDR3 motif is important in antigen recognition is confirmed by the crystal structure of a gammadelta TCR bound to its ligand. The significance of these observations is discussed in the context of gammadelta T-cell biology.

Runx3 regulates dendritic epidermal T cell development

The Runx3 transcription factor regulates development of T cells during thymopoiesis and TrkC sensory neurons during dorsal root ganglia neurogenesis. It also mediates transforming growth factor-beta signaling in dendritic cells and is essential for development of skin Langerhans cells. Here, we report that Runx3 is involved in the development of skin dendritic epidermal T cells (DETCs); an important component of tissue immunoregulation. In developing DETCs, Runx3 regulates expression of the alphaEbeta7 integrin CD103, known to affect migration and epithelial retention of DETCs. It also regulates expression of IL-2 receptor beta (IL-2Rbeta) that mediates cell proliferation in response to IL-2 or IL-15. In the absence of Runx3, the reduction in CD103 and IL-2Rbeta expression on Runx3(-/-) DETC precursors resulted in impaired cell proliferation and maturation, leading to complete lack of skin DETCs in Runx3(-/-) mice. The data demonstrate the requirement of Runx3 for DETCs development and underscore the importance of CD103 and IL-2Rbeta in this process. Of note, while Runx3(-/-) mice lack both DETCs and Langerhans cells, the two most important components of skin immune surveillance, the mice did not develop skin lesions under pathogen-free (SPF) conditions.

Coordination between CCR7- and CCR9-mediated chemokine signals in prevascular fetal thymus colonization

Thymus seeding by T-lymphoid progenitor cells is a prerequisite for T-cell development. However, molecules guiding thymus colonization and their roles before and after thymus vascularization are unclear. Here we show that mice doubly deficient for chemokine receptors CCR7 and CCR9 were defective specifically in fetal thymus colonization before, but not after, thymus vascularization. The defective prevascular fetal thymus colonization was followed by selective loss of the first wave of T-cell development generating epidermal Vγ3+ γδ T cells. Unexpectedly, CCL21, a CCR7 ligand, was expressed not by Foxn1-dependent thymic primordium but by Gcm2-dependent parathyroid primordium, whereas CCL25, a CCR9 ligand, was predominantly expressed by Foxn1-dependent thymic primordium, revealing the role of the adjacent parathyroid in guiding fetal thymus colonization. These results indicate coordination between Gcm2-dependent parathyroid and Foxn1-dependent thymic primordia in establishing CCL21/CCR7- and CCL25/CCR9-mediated chemokine guidance essential for prevascular fetal thymus colonization.

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.

Visualization of the earliest steps of γδ T cell development in the adult thymus

The checkpoint in gammadelta cell development that controls successful T cell receptor (TCR) gene rearrangements remains poorly characterized. Using mice expressing a reporter gene 'knocked into' the Tcrd constant region gene, we have characterized many of the events that mark the life of early gammadelta cells in the adult thymus. We identify the developmental stage during which the Tcrd locus 'opens' in early T cell progenitors and show that a single checkpoint controls gammadelta cell development during the penultimate CD4- CD8- stage. Passage through this checkpoint required the assembly of gammadelta TCR heterodimers on the cell surface and signaling via the Lat adaptor protein. In addition, we show that gammadelta selection triggered a phase of sustained proliferation similar to that induced by the pre-TCR.

T cells in cryptopatch aggregates share TCR gamma variable region junctional sequences with gamma delta T cells in the small intestinal epithelium of mice

The role of cryptopatch aggregates in the development of intestinal intraepithelial lymphocytes (IEL) is a matter of controversy. Therefore, an important question is whether T cells in cryptopatch aggregates are lineally related to IEL. We hypothesized that if gammadelta+ IEL derive from T cells in cryptopatch aggregates, then a clonal relationship would exist between the two populations. To test this hypothesis, we compared the sequence of rearranged TCR gamma variable region 5 genes in gammadelta+ IEL and cryptopatch cells. We purified IEL by FACS and cryptopatch cells were isolated from frozen sections of the intestine by laser-assisted microdissection. PCR showed that TCR gamma variable region 5 was rearranged in gammadelta+ IEL and in CD3+ cryptopatch cells, but not in CD3- cryptopatch cells. DNA sequence analysis showed that the frequency of in-frame junctions in cryptopatch aggregates was at a level consistent with positive selection in both wild-type and athymic nude mice. In addition, the predicted amino acid sequences of V-J junctions present in gammadelta+ IEL and cryptopatch cells were encoded by identical nucleotide sequences. By contrast, the frequency of in-frame joints was significantly reduced in cryptopatch cells isolated from TCR delta-deficient mice, indicating that the enrichment of in-frame joints in cryptopatch cells must normally depend on expression of surface gammadelta TCR. Our results are consistent with the hypothesis that a subset of gammadelta+ IEL are related to T cells in cryptopatch aggregates. The precise role of cryptopatch aggregates in intestinal gammadelta+ T cell homeostasis still needs to be determined.

Epithelial and mucosal gamma delta T cells

Although they constitute a small part of the circulating lymphocyte population, gammadelta T cells are found in high abundance on mucosal and epithelial surfaces. These gammadelta T cells are activated in response to stress to the surrounding tissue and perform a number of functions depending upon the location and type of stress that has occurred. Roles elucidated recently for gammadelta T cells include modulation of epithelial homeostasis through insulin-like growth factor-1 and keratinocyte growth factor, lysis of cytomegalovirus-infected cells, and recruitment of inflammatory cells to sites of tissue damage. Recent advances have provided an understanding of the development of mucosal and skin gammadelta T cells and their roles in restoring and maintaining tissue integrity.

Selection of the cutaneous intraepithelial gammadelta+ T cell repertoire by a thymic stromal determinant

Intraepithelial lymphocytes constitute a group of T cells that express mainly monospecific or oligoclonal T cell receptors (TCRs). Like adaptive TCR alphabeta+ T cells, intraepithelial lymphocytes, a subset enriched in TCR gammadelta+ T cells, are proposed to be positively selected by thymically expressed self agonists, yet no direct evidence for this exists at present. Mouse dendritic epidermal T cells are prototypic intraepithelial lymphocytes, displaying an almost monoclonal TCR gammadelta+ repertoire. Here we describe an FVB substrain of mice in which this repertoire was uniquely depleted, resulting in cutaneous pathology. This phenotype was due to failure of dendritic epidermal T cell progenitors to mature because of a heritable defect in a dominant gene used by the thymic stroma to 'educate' the natural, skin-associated intraepithelial lymphocyte repertoire to be of physiological use.

NKG2D Ligation without T Cell Receptor Engagement Triggers Both Cytotoxicity and Cytokine Production in Dendritic Epidermal T Cells

NKG2D is an activating receptor that recognizes self-ligands induced on stressed, infected, or transformed cells. In mice, two NKG2D isoforms (NKG2D-S (short) and NKG2D-L (long)) that associate differentially with DAP10 and DAP12 adaptor proteins exist. Differential expression of these isoforms and adaptor proteins depending on the activating state and cell types determines distinct functional outcomes of NKG2D ligation: direct activation of cytotoxicity in natural killer (NK) cells and cytokine production in activated NK cells, but only costimulation in activated CD8+ T cells. Intraepithelial gammadelta T cells of the mouse skin, termed dendritic epidermal T cells (DETCs), were also shown to express NKG2D, but the NKG2D isoform(s) expressed in DETCs have not been determined. Furthermore, functional outcomes of NKG2D ligation in DETCs are largely unknown, although costimulation of DETC-mediated cytotoxicity by NKG2D was demonstrated. Here, we show that DETCs constitutively express NKG2D-S, NKG2D-L, DAP10, and DAP12 transcripts as well as cell surface NKG2D protein. Blocking of NKG2D inhibited DETC-mediated cytotoxicity against target cells that do not express T cell receptor ligands. Cross-linking of NKG2D on DETCs induced IFN-gamma production. These findings demonstrate that DETCs constitutively express NKG2D that acts as a primary activating receptor, and indicate its important role in cutaneous immune surveillance.

Interleukin 15 controls the generation of the restricted T cell receptor repertoire of gamma delta intestinal intraepithelial lymphocytes

The gammadelta T cells are prevalent in the mucosal epithelia and are postulated to act as 'sentries' for maintaining tissue integrity. What these gammadelta T cells recognize is poorly defined, but given the restricted T cell receptor (TCR) repertoire, the idea that they are selected by self antigens of low complexity has been widely disseminated. Here we present data showing that the generation of the restricted TCR variable gamma-region gene repertoire of intestinal intraepithelial lymphocytes was regulated by interleukin 15, which induced local chromatin modifications specific for the variable gamma-region gene segment and enhanced accessibility conducive to subsequent targeted gene rearrangement. This cytokine-directed tissue-specific TCR repertoire formation probably reflects distinct TCR repertoire selection criteria for gammadelta and alphabeta T cell lineages adopted for different antigen-recognition strategies.

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.

Characterization of the T-cell receptor gamma locus and analysis of the variable gene segment expression in rabbit

The genomic organization and expression of genes of the T-cell receptor gamma (TRG) locus are described for mice and humans, but not for species such as rabbits (Oryctolagus cuniculus), in which gammadelta T cells compose a sizeable proportion of T cells in the periphery. We cloned 200 kb of the rabbit TRG locus and determined the TRGV gene usage in adult and newborn rabbits by RT-PCR. We identified two TRGJ genes, one TRGC gene, and 22 TRGV genes, all of which encoded functional variable regions. One TRGV gene is the unique member of the TRGV2 subgroup, whereas the other genes belong to the TRGV1 subgroup. Evolutionary analyses of TRGV1 genes identified three distinct groups that can be explained by separate duplication events in the rabbit genome. Evidence of gene conversion between TRGV1.1 and TRGV1.6 was observed. Both TRGV1 and TRGV2 subgroup genes were expressed in the spleen, intestine, and appendix of adult rabbits, and the repertoire of TRGV genes expressed in these tissues was similar. In these tissues from newborns, and in skin from adults, only the genes from the TRGV1 subgroup were expressed. Greater TRGV-J junctional diversity was found in tissues from adult compared to newborn rabbits. Our analyses indicate rabbits have a larger germ line encoded TRG repertoire compared with that of mice and humans. In addition, we found TRGV gene usage is alike in most tissues of rabbits similar to that found in humans but in contrast to that found in mice.

The fourth way? Harnessing aggressive tendencies in the thymus

During late stages of thymic development, T cells must chose between different fates, dictated by their TCR specificity. Typically, this is thought of as a choice between three alternatives (being positive selection for useful T cells vs negative selection or neglect for harmful or useless T cells). However, there is growing evidence for a fourth alternative, in which T cells are positively selected by agonist ligands, which would normally be expected to induce T cell deletion. In this review, we will discuss where and when agonist selection is induced and whether this should be considered as a novel form of thymic selection or as an alternative differentiation state for Ag-exposed T cells.