Requirement of the IL-2 receptor beta chain for the development of Vgamma3 dendritic epidermal T cells

Skin barrier immunology from early life to adulthood

Our skin has a unique barrier function, which is imperative for the body's protection against external pathogens and environmental insults. Although interacting closely and sharing many similarities with key mucosal barrier sites, such as the gut and the lung, the skin also provides protection for internal tissues and organs and has a distinct lipid and chemical composition. Skin immunity develops over time and is influenced by a multiplicity of different factors, including lifestyle, genetics, and environmental exposures. Alterations in early life skin immune and structural development may have long-term consequences for skin health. In this review, we summarize the current knowledge on cutaneous barrier and immune development from early life to adulthood, with an overview of skin physiology and immune responses. We specifically highlight the influence of the skin microenvironment and other host intrinsic, host extrinsic (e.g. skin microbiome), and environmental factors on early life cutaneous immunity.

Vγ5Vδ1 TCR signaling is required to different extents for embryonic versus postnatal development of DETCs

δγ T cells expressing Vγ5Vδ1 TCR originally develop in the embryonic thymus and migrate to the epidermis, forming dendritic epidermal T cells (DETCs) throughout life. It is thought that a TCR signal is essential for their development; e.g., lack of TCR signal-transducer ZAP70 significantly decreases DETC numbers. On the other hand, lack of ZAP70 does not affect Vγ5Vδ1 + T cells in the embryonic thymus; thus, the involvement of TCR signaling remains elusive. Here, we used SKG mice with attenuated TCR signaling rather than gene-knockout mice. In SKG mice, Vγ5 + T cells showed a marked decrease (10% of wild-type) in adult epidermis; however, there was just a moderate decrease (50% of wild-type) in the embryonic thymus. In early postnatal epidermis in SKG mice, substantial numbers of Vγ5 + T cells were observed (50% of wild-type). Their activation markers including CD122, a component of the IL-15 receptor indispensable for DETC proliferation, were comparable to those of WT. However, the Vγ5 + T cells in SKG mice did not proliferate and form DETCs thereafter. Furthermore, in SKG/+ mice, the number of thymic Vγ5Vδ1 + T cells increased, compared to SKG mice; however, the number of DETCs remained significantly lower than in WT, similar to SKG mice. Our results suggest that signaling via Vγ5Vδ1 TCR is indispensable for DETC development, with distinct contributions to embryonic development and postnatal proliferation.

αβ T cells replacing dermal and epidermal γδ T cells in Tcrd-/- mice express an MHC-independent TCR repertoire

The epidermis of mouse skin is usually populated by dendritic epidermal T cells (γδDETC) expressing an invariant Vγ5Vδ1⁺ TCR. In Tcrd^-/- mice, skin-resident γδDETC are replaced by αβDETC carrying polyclonal αβ TCRs. Although they exhibit a dendritic morphology, αβDETC were reported to be less functional than genuine γδDETC, likely because their TCR is unable to interact with the original TCR ligands of γδDETC. However, the TCR repertoire of those replacement DETC in Tcrd^-/- mice might provide clues for understanding the development and selection of canonical γδDETC. Here, we compare the phenotype and TCR repertoires of wild-type and Tcrd^-/- mouse skin T cells. Our data reveal that αβDETC are CD4/CD8 double negative and express an MHC-independent TCR repertoire. Furthermore, we identify a second MHC-independent population of CD103^hi CD4/ CD8 double-negative αβ T cells in the dermis of Tcrd^-/- mice.

The molecular mechanisms supporting the homeostasis and activation of dendritic epidermal T cell and its role in promoting wound healing

The epidermis is the outermost layer of skin and the first barrier against invasion. Dendritic epidermal T cells (DETCs) are a subset of γδ T cells and an important component of the epidermal immune microenvironment. DETCs are involved in skin wound healing, malignancy and autoimmune diseases. DETCs secrete insulin-like growth factor-1 and keratinocyte growth factor for skin homeostasis and re-epithelization and release inflammatory factors to adjust the inflammatory microenvironment of wound healing. Therefore, an understanding of their development, activation and correlative signalling pathways is indispensable for the regulation of DETCs to accelerate wound healing. Our review focuses on the above-mentioned molecular mechanisms to provide a general research framework to regulate and control the function of DETCs.

Bibliometric Analysis of Dendritic Epidermal T Cell (DETC) Research From 1983 to 2019

Dendritic epidermal T cells (DETC) are a group of immune cells expressing canonical γδ TCR in the murine epidermis. Similar to γδ T cells in the human epidermis, DETC serve an important barrier cell in the skin and participate in skin immune surveillance, immune regulation, skin homeostasis, tissue protection, and other activities. Since its discovery in 1983, research on DETC has grown rapidly and unevenly. To evaluate DETC research trends and map the DETC knowledge structure, we have applied bibliometric methods and techniques. A total of 384 DETC-related articles obtained from the Scopus database published between 1983 and 2019 were analyzed using indicators of publication and citation metrics, country and international cooperation, author and co-authorship, and keyword co-occurrence cluster. The present research status, the emerging global trends and the future development direction are also visualized and discussed. In summary, this study provides novel and useful data for the DETC research scientific community, and will help researchers explore DETC more intuitively and effectively.

Isolation and Analysis of Mouse and Human Skin γδ T Cells

Skin-resident and infiltrating γδ T lymphocytes are components of the cutaneous immune system that provide the first line of defense against pathogens and the environment. Research that employs the isolation and culture of T cells from murine and human skin can help delineate the molecular and cellular mechanisms utilized by T lymphocytes in skin-specific immunity. However, obtaining high numbers of T cells from epithelial tissue without resorting to long-term culture or transformation can be difficult. Here, specific approaches are described for the isolation and culture of γδ T lymphocytes from murine skin and human skin explant cultures. In addition, a protocol to assess the morphology and activation of epidermal γδ T cells in situ using immunofluorescent microscopy is detailed. These techniques can be used to analyze resident and infiltrating γδ T lymphocytes in the skin via flow cytometry, RNA-seq, or proteomics to further study inflammatory diseases, cancer, or autoimmunity. © 2019 by John Wiley & Sons, Inc. Basic Protocol 1: Isolation, culture, and analysis of γδ T cells from murine epidermis Basic Protocol 2: Examination of γδ T cells in epidermal sheets to assess activation and morphology Basic Protocol 3: Preparation of human skin explant cultures for analysis of skin T cells Support Protocol 1: Counting live cells with hemocytometer Support Protocol 2: Preparing a Matrigel.

Tissue Adaptations of Memory and Tissue-Resident Gamma Delta T Cells

Epithelial and mucosal barriers are critical interfaces physically separating the body from the outside environment and are the tissues most exposed to microorganisms and potential inflammatory agents. The integrity of these tissues requires fine tuning of the local immune system to enable the efficient elimination of invasive pathogens while simultaneously preserving a beneficial relationship with commensal organisms and preventing autoimmunity. Although they only represent a small fraction of circulating and lymphoid T cells, γδ T cells form a substantial population at barrier sites and even outnumber conventional αβ T cells in some tissues. After their egress from the thymus, several γδ T cell subsets naturally establish residency in predetermined mucosal and epithelial locations, as exemplified by the restricted location of murine Vγ5⁺ and Vγ3Vδ1⁺ T cell subsets to the intestinal epithelium and epidermis, respectively. Because of their preferential location in barrier sites, γδ T cells are often directly or indirectly influenced by the microbiota or the pathogens that invade these sites. More recently, a growing body of studies have shown that γδ T cells form long-lived memory populations upon local inflammation or bacterial infection, some of which permanently populate the affected tissues after pathogen clearance or resolution of inflammation. Natural and induced resident γδ T cells have been implicated in many beneficial processes such as tissue homeostasis and pathogen control, but their presence may also exacerbate local inflammation under certain circumstances. Further understanding of the biology and role of these unconventional resident T cells in homeostasis and disease may shed light on potentially novel vaccines and therapies.

IL-15 Enhances Activation and IGF-1 Production of Dendritic Epidermal T Cells to Promote Wound Healing in Diabetic Mice

Altered homeostasis and dysfunction of dendritic epidermal T cells (DETCs) contribute to abnormal diabetic wound healing. IL-15 plays important roles in survival and activation of T lymphocytes. Recently, reduction of epidermal IL-15 has been reported as an important mechanism for abnormal DETC homeostasis in streptozotocin -induced diabetic animals. However, the role of IL-15 in impaired diabetic wound healing remains unknown. Here, we found that, through rescuing the insufficient activation of DETCs, IL-15 increased IGF-1 production by DETCs and thereby promoted diabetic skin wound repair. Regulation of IGF-1 in DETCs by IL-15 was partly dependent on the mTOR pathway. In addition, expression of IL-15 and IGF-1 were positively correlated in wounded epidermis. Together, our data indicated that IL-15 enhanced IGF-1 production by DETCs to promoting diabetic wound repair, suggesting IL-15 as a potential therapeutic agent for managing diabetic wound healing.

γδ T cells in homeostasis and host defence of epithelial barrier tissues

Epithelial surfaces line the body and provide a crucial interface between the body and the external environment. Tissue-resident epithelial γδ T cells represent a major T cell population in the epithelial tissues and are ideally positioned to carry out barrier surveillance and aid in tissue homeostasis and repair. In this Review, we focus on the intraepithelial γδ T cell compartment of the two largest epithelial tissues in the body - namely, the epidermis and the intestine - and provide a comprehensive overview of the crucial contributions of intraepithelial γδ T cells to tissue integrity and repair, host homeostasis and protection in the context of the symbiotic relationship with the microbiome and during pathogen clearance. Finally, we describe epithelium-specific butyrophilin-like molecules and briefly review their emerging role in selectively shaping and regulating epidermal and intestinal γδ T cell repertoires.

Interleukin-15 as a potential new target in Sjögren's syndrome-associated inflammation

IL-15 is a key regulatory cytokine that shares many biological properties with IL-2. Recently, it has been shown that IL-15 could be up-regulated in T cell-mediated inflammatory disorders, such as rheumatoid arthritis and inflammatory bowel diseases. However, the role and expression of IL-15 in the inflammatory autoimmune disease Sjögren's syndrome (SS) has not been investigated. In the present study we evaluated the expression of IL-15 mRNA and protein in minor salivary gland (MSG) biopsy specimens and in human salivary gland epithelial cell (SGEC) cultures obtained from patients with primary SS (pSS) and compared their expression with that seen in normal healthy control subjects. IL-15 gene and protein analysis revealed that SGEC are able to produce IL-15. Results obtained demonstrated that the number of IL-15(+) cultured SGEC was significantly higher in cells derived from patients with pSS in comparison with SGEC from healthy subjects; similar results were obtained for IL-15 immunoreactivity by using immunohistochemistry that revealed a strong expression both in acinar and in ductal cells from pSS MSG. These studies could provide a rational basis to determine whether IL-15 could be a good candidate for anti-cytokine therapy in chronic inflammatory pSS diseases.

New Molecular and Cellular Mechanisms of Tolerance: Tolerogenic Actions of IL-2

Interleukin-2 (IL-2) is an old molecule with brand new functions. Indeed, IL-2 has been first described as a T-cell growth factor but recent data pointed out that its main function in vivo is the maintenance of immune tolerance. Mechanistically, IL-2 is essential for the development and function of CD4(+) Foxp3(+) regulatory T cells (Treg cells) that are essential players in the control of immune responded to self, tumors, microbes and grafts. Treg cells are exquisitely sensitive to IL-2 due to their constitutive expression of the high affinity IL-2 receptor (IL-2R) and the new paradigm suggests that low-doses of IL-2 could selectively boost Treg cells in vivo. Consequently, a growing body of clinical research is aiming at using IL-2 at low doses as a tolerogenic drug to boost endogenous Treg cells in patients suffering from autoimmune or inflammatory conditions. In this manuscript, we briefly review IL-2/IL-2R biology and the role of IL-2 in the development, maintenance, and function of Treg cells; and also its effects on other immune cell populations such as CD4(+) T helper cells and CD8(+) memory T cells. Then, focusing on type 1 diabetes, we review the preclinical studies and clinical trials supporting the use of low-doses IL-2 as a tolerogenic immunotherapy. Finally, we discuss the limitations and future directions for IL-2 based immunotherapy.

Programmed downregulation of CCR6 is important for establishment of epidermal γδT cells by regulating their thymic egress and epidermal location

The skin as the outmost epithelial tissue is under frequent physical, chemical, and biological assaults. To counter the assaults and maintain the local tissue homeostasis, the skin is stationed with various innate or innate-like lymphocytes such as γδT cells. Increasing evidence suggests that an intrathymically programmed process is involved in coordinated expression of multiple homing molecules on specific γδT cell subsets to direct their localization in different regions of the skin for the protective functions. However, detailed molecular events underlying the programmed skin distribution of specific γδT cell subsets are not fully understood. We report in this study that the temporally and spatially regulated downregulation of chemokine receptor CCR6 on fetal thymic Vγ3(+) epidermal γδT precursors is involved in their thymic egress and proper localization in the epidermis. Failure of downregulation of CCR6 in the mature Vγ3(+) epidermal γδT precursor cells due to the constitutive expression of transgenic CCR6 resulted in their abnormal accumulation in the fetal thymus and reduced numbers of the epidermal γδT cells. In addition, the transgenic expression of CCR6 on the Vγ3(+) γδT cells also improperly increased their distribution in dermis of the skin. Those findings advanced our understanding of the molecular basis regulating the tissue specific distribution of various innate-like γδT cell lymphocytes in the skin.

CCR10 and its ligands in regulation of epithelial immunity and diseases

Epithelial tissues covering the external and internal surface of a body are constantly under physical, chemical or biological assaults. To protect the epithelial tissues and maintain their homeostasis, multiple layers of immune defense mechanisms are required. Besides the epithelial tissue-resident immune cells that provide the first line of defense, circulating immune cells are also recruited into the local tissues in response to challenges. Chemokines and chemokine receptors regulate tissue-specific migration, maintenance and functions of immune cells. Among them, chemokine receptor CCR10 and its ligands chemokines CCL27 and CCL28 are uniquely involved in the epithelial immunity. CCL27 is expressed predominantly in the skin by keratinocytes while CCL28 is expressed by epithelial cells of various mucosal tissues. CCR10 is expressed by various subsets of innate-like T cells that are programmed to localize to the skin during their developmental processes in the thymus. Circulating T cells might be imprinted by skin-associated antigen- presenting cells to express CCR10 for their recruitment to the skin during the local immune response. On the other hand, IgA antibody-producing B cells generated in mucosa-associated lymphoid tissues express CCR10 for their migration and maintenance at mucosal sites. Increasing evidence also found that CCR10/ligands are involved in regulation of other immune cells in epithelial immunity and are frequently exploited by epithelium-localizing or -originated cancer cells for their survival, proliferation and evasion from immune surveillance. Herein, we review current knowledge on roles of CCR10/ligands in regulation of epithelial immunity and diseases and speculate on related important questions worth further investigation.

Involvement of an NKG2D ligand H60c in epidermal dendritic T cell-mediated wound repair

Dendritic epidermal T cells (DETCs) found in mouse skin are NKG2D-positive γδ T cells involved in immune surveillance and wound repair. It is assumed that the interaction of an NKG2D receptor on DETCs and an MHC class I-like NKG2D ligand on keratinocytes activates DETCs, which then secrete cytokines promoting wound repair. However, direct evidence that DETC activation through NKG2D signaling promotes wound repair is not available. In the present study, we generated mAbs for an NKG2D ligand H60c previously suggested to be expressed specifically on skin keratinocytes. Local administration of H60c-specific mAb inhibited activation of DETCs and significantly delayed wound repair. Likewise, administration of NKG2D-specific mAb impaired wound repair to a similar extent. The delay in wound closure resulting from the blockade of the NKG2D pathway was comparable to that observed in γδ T cell-deficient mice. These results indicate that H60c/NKG2D interactions play a critical role in wound repair. Reassessment of binding affinities showed that H60c monomers bind to NKG2D with affinity (K(d) = 26 ± 3.2 nM) comparable to those of other high-affinity NKG2D ligands. H60c is transcribed not only in skin but also in tissues such as tongue and female reproductive tract known to contain epithelium-resident γδ T cells expressing invariant TCRs, suggesting a more general role for H60c in the maintenance of epithelial integrity.

Role for E-cadherin as an inhibitory receptor on epidermal gammadelta T cells

E-cadherin is a homophilic adhesion molecule that maintains homotypic intercellular adhesion between epithelial cells such as epidermal keratinocytes. E-cadherin is also expressed on resident murine epidermal γδ T cells, known as dendritic epidermal T cells (DETCs), but they express another receptor for E-cadherin, α(E)(CD103)β(7) integrin, as well. In this study, we analyzed functional differences between E-cadherin-mediated homophilic binding and heterophilic binding of α(E)β(7) integrin to E-cadherin in heterotypic intercellular adhesion of DETCs to keratinocytes. E-cadherin, but not α(E)β(7) integrin, was downregulated on activation of DETCs in vivo and in vitro. Short-term (1-h) adhesion of DETCs to keratinocytes in vitro was primarily mediated by α(E)β(7) integrin, and blocking of the binding of α(E)β(7) integrin to E-cadherin inhibited the lysis of keratinocytes by DETCs. Stable binding of E-cadherin on DETCs to plate-bound recombinant E-cadherin was observed only after 24-h culture in vitro. Cytokine production and degranulation by DETCs in response to suboptimal TCR cross-linking and mitogen stimulation were augmented by coligation of α(E)β(7) integrin. In contrast, engagement of E-cadherin on DETCs with immobilized anti-E-cadherin Ab, plate-bound recombinant E-cadherin, and E-cadherin on keratinocytes inhibited DETC activation. Therefore, E-cadherin acts as an inhibitory receptor on DETCs, whereas α(E)β(7) integrin acts as a costimulatory receptor. Differential expression of E-cadherin and α(E)β(7) integrin on resting and activated DETCs, as well as their opposite functions in DETC activation, suggests that E-cadherin and α(E)β(7) integrin on DETCs regulate their activation threshold through binding to E-cadherin on keratinocytes.

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.

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.

The biology of interleukin-2

Much data support an essential role for interleukin (IL)-2 in immune tolerance. This idea is much different from the early paradigm in which IL-2 is central for protective immune responses. This change in thinking occurred when a T regulatory cell defect was shown to be responsible for the lethal autoimmunity associated with IL-2/IL-2R deficiency. This realization allowed investigators to explore immune responses in IL-2-nonresponsive mice rendered autoimmune-free. Such studies established that IL-2 sometimes contributes to optimal primary immune responses, but it is not mandatory. Emerging findings, however, suggest an essential role for IL-2 in immune memory. Here, the current understanding of the dual role of IL-2 in maintaining tolerance and contributing to immunity in vivo is reviewed with some emphasis on T regulatory cell production and homeostasis. Also discussed are implications of this new appreciation concerning the immunobiology of IL-2 with respect to targeting IL-2 or its receptor in immunotherapy.

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.

Enforced Expression of Bcl-2 Partially Restores Cell Numbers but Not Functions of TCRγδ Intestinal Intraepithelial T Lymphocytes in IL-15-Deficient Mice

IL-15 knockout (KO) mice have severely reduced numbers of TCRgammadelta intestinal intraepithelial T lymphocytes (i-IEL), suggesting requirements of IL-15 signaling in the development or maintenance of i-IEL. To determine an involvement of survival signals via Bcl-2 in IL-15-mediated homeostasis of TCRgammadelta i-IEL, we introduced a bcl-2 transgene into IL-15 KO mice. In situ apoptosis of TCRgammadelta i-IEL was decreased in Bcl-2 transgenic (Tg) x IL-15 KO mice compared with IL-15 KO mice. The enforced expression of Bcl-2 partially restored the numbers of TCRgammadelta i-IEL in IL-15 KO mice. However, effector functions of TCRgammadelta i-IEL, including cytokine production and cytotoxic activity, were not recovered in Bcl-2 Tg x IL-15 KO mice. Importantly, TCRgammadelta i-IEL in Bcl-2 Tg x IL-15 KO mice expressed a reduced level of eomesodermin, a transcription factor critical for effector functions of NK cells and CD8(+) T cells. Similar to the case of TCRgammadelta i-IEL, enforced expression of Bcl-2 restored the numbers but not the functions of NK cells in IL-15 KO mice. These results suggest that Bcl-2-mediated survival signal is involved in the IL-15-mediated homeostasis of TCRgammadelta i-IEL and NK cells, but other signals from IL-15 are critical for inducing transcription factors, such as eomesodermin for their effector functions.

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.

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.

Up-regulation of cell surface Toll-like receptor 4-MD2 expression on dendritic epidermal T cells after the emigration from epidermis during cutaneous inflammation

BACKGROUND

Mouse epidermis contains a population of gammadelta T cells, termed dendritic epidermal T cells (DETCs), which uniformly express the invariant Vgamma3 T cell receptor. Certain DETC lines were reported to respond to Gram-negative bacteria in the presence of immobilized anti-CD3 monoclonal antibody or to lipopolysaccharide (LPS) in the presence of B cell lines.

OBJECTIVE

To determine whether DETCs express the primary signaling receptor for LPS, Toll-like receptor (TLR) 4-MD2.

METHODS

We analyzed expression of TLR4-MD2 in three independent DETC lines as well as in freshly isolated DETCs.

RESULTS

All DETC lines expressed TLR4 and MD2 transcripts and TLR4-MD2 protein complex intracellularly, but none expressed TLR4-MD2 on the cell surface. By immunoblotting, only the immature form of TLR4 protein was detected in the DETC lines. The DETC lines did not respond to LPS even in the presence of immobilized anti-CD3 monoclonal antibody. Freshly isolated DETCs and their fetal thymic precursors also lacked cell surface expression of TLR4-MD2, but a small subpopulation of dermal Vgamma3 T cells isolated from croton oil-painted skin expressed TLR4-MD2 on the cell surface. Similarly, Vgamma3 T cells emigrated from organ-cultured epidermis expressed cell surface TLR4-MD2.

CONCLUSIONS

These results demonstrate that DETCs do not constitutively express cell surface TLR4-MD2, but TLR4-MD2 expression may be up-regulated when DETCs emigrate from epidermis during cutaneous inflammation.

STAT5 is required for thymopoiesis in a development stage-specific manner

Diverse cytokines necessary for normal lymphopoiesis and lymphocyte homeostasis activate STAT5 in responder cells. Although STAT5 has been suggested to be a central molecular effecter of IL-7 function, its essential role during IL-7-dependent T cell development in vivo remained unclear. Using Stat5(-/-) mice we now show that STAT5 is essential for various functions ascribed to IL-7 in vivo. STAT5 is required for embryonic thymocyte production, TCRgamma gene transcription, and Peyer's patch development. In sharp contrast, normal STAT5 is dispensable for adult thymopoiesis. In peripheral lymphocytes, STAT5 is primarily required for the generation and/or maintenance of gammadelta T cells and TCRgammadelta(+) intraepithelial lymphocytes. Collectively, these results demonstrate that STAT5 is critical for many, but not all, aspects of steady state lymphoid lineage development and maintenance and suggest the existence of previously undocumented cytokine signaling traits and/or cytokine milieu during adult thymopoiesis.

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

The role of cellular selection in the development of gammadelta T cells remains unclear. Knockout mice lacking a subset of Vgamma genes, including Vgamma3, contain abundant gammadelta T cells but are devoid of dendritic epidermal gammadeltaT cells (DETCs), which normally express an invariant Vgamma3/Vdelta1 gammadelta TCR. A rearranged Vgamma2 transgene restored DETC development, but the restored DETCs selectively expressed a unique Vdelta gene other than Vdelta1, indicating that DETC development involves TCR-based selection. In both normal and transgenic/knockout mice, specific DETC precursors in the fetal thymus were activated and expressed the IL-15 receptor beta chain, skin-homing receptors, and thymic exiting receptors. In vitro activation of irrelevant precursors also led to upregulation of the skin-homing receptor, providing an explanation for how thymic selection is coordinated with development of epidermal gammadelta T cells.

Expression of functional Toll-like receptor 2 on human epidermal keratinocytes

Epidermal keratinocytes secrete cytokines, chemokines, and anti-microbial peptides in response to various microbial pathogens and their components including lipopolysaccharide (LPS). To identify the receptor(s) involved in the anti-microbial responses of epidermal keratinocytes, we analyzed expression of CD14, Toll-like receptor 2 (TLR2), and TLR4 on cultured normal human epidermal keratinocytes (NHEK). Although CD14 and TLR2 mRNA were expressed in cultured NHEK, only TLR2 was detected on the cell surface. Cultured NHEK did not express TLR4 mRNA or protein. Commercial LPS preparations could stimulate epidermal keratinocytes to produce beta-defensin-2 and IL-8, and the LPS response was inhibited with mAb specific for TLR2, but not for CD14 or TLR4. Repurified LPS and lipid A did not stimulate epidermal keratinocytes, whereas peptidoglycan (PGN) from Gram-positive bacteria and yeast cell wall particle induced beta-defensin-2 and IL-8 production. Thus, cultured NHEK express functional TLR2, but not CD14 or TLR4, and the "LPS" response of epidermal keratinocytes shown in the previous studies might be mediated by TLR2-dependent recognition of non-LPS bacterial components contaminating in commercial LPS preparations. In the normal human skin, however, epidermal keratinocytes expressed both TLR2 and TLR4. Because TLR4 was induced in epidermal keratinocytes by in vitro stimulation with PGN from Gram-positive bacteria, constitutive expression of TLR4 on epidermal keratinocytes in vivo might also be induced by continuous recognition of the resident skin flora containing Gram-positive bacteria through TLR2.

Enforced expression of Bcl-2 restores the number of NK cells, but does not rescue the impaired development of NKT cells or intraepithelial lymphocytes, in IL-2/IL-15 receptor beta-chain-deficient mice

IL-2/IL-15Rbeta-deficient mice display impaired development of NK cells, NKT cells, and intraepithelial lymphocytes of the intestine and skin. To determine the role of survival signals mediated by IL-2/IL-15R in the development of these innate lymphocytes, we introduced a bcl-2 transgene into IL-2/IL-15Rbeta-deficient mice. Enforced expression of Bcl-2 restored the number of NK cells in IL-2/IL-15Rbeta-deficient mice, but the rescued NK cells showed no cytotoxic activity. The numbers of NKT cells and intestinal intraepithelial lymphocytes did not increase significantly, and skin intraepithelial lymphocytes remained undetectable in the bcl-2 transgenic IL-2/IL-15Rbeta-deficient mice. These results indicate an essential role of IL-2/IL-15R-mediated survival signals in the development of NK cells, but they also show that additional nonsurvival signals from IL-2/IL-15R are necessary for innate lymphocyte development.

Developmental and functional defects of thymic and epidermal V gamma 3 cells in IL-15-deficient and IFN regulatory factor-1-deficient mice

In this study, the role of IL-15 and its regulation by the transcription factor IFN regulatory factor-1 (IRF-1) in murine V gamma 3 T cell development and activity is assessed. Compared with wild-type (WT) mice, reduced numbers of mature V gamma 3 cells were found in the fetal thymus of IL-15(-/-) mice, while IRF-1(-/-) mice displayed normal frequencies. V gamma 3(+) dendritic epidermal T cells (DETCs) were absent in IL-15(-/-) mice but present in IRF-1(-/-) mice. DETCs from IRF-1(-/-) mice displayed morphologically a less mature phenotype and showed different emergence kinetics during ontogeny. This corresponded with lower IL-15 mRNA levels in the skin epidermis. Comparable levels of IL-7 were found in the skin of WT and IL-15(-/-) mice. Adoptive transfer experiments of WT fetal thymocytes into IL-15(-/-) mice did not result in the development of V gamma 3(+) DETCs, confirming the nonredundant role of IL-15 in the skin during DETC development. In vitro, cytolytic activity of IL-15(-/-) V gamma 3 cells was normal after stimulation with IL-15 and was further enhanced by addition of IL-12. In contrast, cytolytic activity of IRF-1(-/-) V gamma 3 cells remained defective after stimulation with IL-15 in combination with IL-12. These data suggest that IL-15 is redundant for the development and/or survival of mature V gamma 3 cells in the fetal thymus, whereas it is essential for the localization of V gamma 3 cells in the skin. Furthermore, a possible role for IRF-1 in inducing morphological maturation of DETCs and cytolytic capacity of V gamma 3 cells is suggested.

Seeding of dendritic epidermal T cells in the neonatal skin is reduced in 129 strain of mice

Precursors for Thy-1(+) dendritic epidermal T cells (DETC) develop as Vgamma3(+) T cells in the fetal thymus and become distributed in the adult skin. DETC are variably distributed from site to site and from strain to strain. To elucidate the basis of strain variation, we first compared the density of DETC in the ear epidermis among different mouse strains. In the ear epidermis, we detected the highest level of DETC in C57BL/6 mice, intermediate levels in C3H and CBA/J mice, and the lowest levels in other strains including BALB/c and 129 mice. Although BALB/c and 129+Ter/Sv mice showed higher levels of DETC in the abdomen than in the ear, the levels were significantly lower than C57BL/6 mice. Furthermore, in neonatal abdominal epidermis we detected considerably lower numbers of DETC in BALB/c and 129+Ter/Sv mice than in C57BL/6 mice. In contrast, Vgamma3(+) DETC precursors in the fetal thymus are rather increased in 129+Ter/Sv mice. These results suggest that fewer DETC precursors are seeded in the neonatal skin of BALB/c and 129+Ter/Sv mice and that their expansion in the skin during neonatal to adult stages does not reach the levels in C57BL/6 mice.

gammadelta T cells express activation markers in the central nervous system of mice with chronic-relapsing experimental autoimmune encephalomyelitis

In this study, we assessed the expression of activation markers on gammadelta T cells in central nervous system (CNS) lesions of SJL mice adoptively sensitized to develop experimental autoimmune encephalomyelitis (EAE) using myelin basic protein-reactive T cells. Although disease expression is known to be dependent upon T cells that express the alphabeta T cell receptor (TCR), a role for gammadelta T cells has been implicated in some studies but not in others. Using three-color flow cytometric analysis of both total and gammadelta T cells in spleen and CNS, the data showed that expression of CD69 (early activation marker), CD62L (lymphocyte homing receptor), CD25 (IL-2Ralpha), CD122 (IL-2Rbeta) and CD95/CD95L (Fas/FasL), fluctuated on gammadelta T cells in EAE lesions in a disease-related fashion. Furthermore, the pattern of expression for these markers on gammadelta T cells was distinct from that found on the total lymphocyte population. Cytokine analysis of gammadelta T cells in the CNS demonstrated a bias towards a Th1-like cytokine profile. From these data, we conclude that gammadelta T cells in EAE lesions display an activated phenotype and form a dynamic component of the total lymphocyte population in the CNS, supporting a contributory role for these cells.

Differential roles of cytokine receptors in the development of epidermal gamma delta T cells

IL-7 and IL-15 play important roles in gammadelta T cell development. These receptors transmit proliferation and/or survival signals in gammadelta T cells. In addition, the IL-7R promotes recombination and transcription in the TCR gamma locus. To clarify the role of the cytokine receptors in the development of epidermal gammadelta T cells, we introduced a Vgamma3/Vdelta1 TCR transgene, derived from Thy-1+ dendritic epidermal T cells (DETC), into IL-7Ralpha-deficient mice, and we found that they partly rescued gammadelta T cells in the adult thymus but not in the spleen. Introduction of an additional Bcl-2 transgene had a minimal effect on gammadelta T cells in the adult thymus of these mice. In contrast to the adult thymus, the introduction of the Vgamma3/Vdelta1 TCR transgene into IL-7Ralpha-/- mice completely restored Vgamma3+ T cells in the fetal thymus and DETC in the adult skin. On the contrary, the same Vgamma3/Vdelta1 TCR transgene failed to rescue DETC in the skin of IL-2Rbeta-deficient mice, even with the additional Bcl-2 transgene. These results suggest that the IL-2/IL-15R, rather than the IL-7R, plays an essential role in proliferation and survival of DETC in the fetal thymus and the skin. In contrast, the IL-7R is probably essential in the induction of V-J recombination of TCRgamma genes. Thus, this study proves that IL-7R and IL-2/IL-15R serve differential functions in epidermal gammadelta T cell development.

Cutaneous DNA vaccination against Ebola virus by particle bombardment: histopathology and alteration of CD3-positive dendritic epidermal cells

We analyzed the localization of gold particles, expression of immunogenic protein, and histopathologic changes after vaccinating guinea pigs and mice with a DNA vaccine to the Ebola virus glycoprotein administered by cutaneous particle bombardment. Gold particles were deposited in all layers of the epidermis and in the dermis. Those in the epidermis were lost as the damaged layers sloughed, while those in the dermis were phagocytized by macrophages. Glycoprotein was demonstrated by immunohistochemistry primarily in keratinocytes in the epidermis and hair follicle epithelium and less frequently in dermal macrophages, fibroblasts, sebocytes, and cells that appeared to be Langerhans cells. The number of cells that expressed glycoprotein increased between 4 and 8 hours postvaccination, then decreased to near zero by 48 hours. The vaccine sites were histologically divisible into three zones. The central portion, zone 1, contained the most gold particles in the dermis and epidermis and had extensive tissue damage, including full-thickness epidermal necrosis. Zone 2 contained fewer gold particles in the epidermis and dermis and had less extensive necrosis. The majority of cells in which glycoprotein was expressed were in zone 2. Zone 3 contained gold particles only in the epidermis and had necrosis of only a few scattered cells. Regeneration of the epidermis in damaged areas was evident at 24 hours postvaccination and was essentially complete by day 5 in the mice and day 10 in the guinea pigs. Inflammatory changes were characterized by hemorrhage, edema, and infiltrates of neutrophils initially and by infiltrates of lymphocytes and macrophages at later times. In zone 1, inflammation affected both the epidermis and dermis. Peripherally, inflammation was relatively limited to the epidermis. CD3-positive dendritic epidermal cells were demonstrated in the epidermis and superficial hair follicles of unvaccinated immunocompetent mice and beige mice but not of SCID mice. These cells disappeared from all but the most peripheral portions of the vaccine sites of vaccinated mice within 24 hours. They reappeared slowly, failing to reach numbers comparable with unvaccinated mice by 35 days postvaccination. The epidermis of control guinea pigs also had CD3-positive cells, but they did not have dendrites. These findings should contribute to a better understanding of the mechanisms operating in response to DNA vaccination by particle bombardment.

Homogeneous epithelial gamma delta T cell repertoire of the skin is shaped through peripheral selection

In contrast to the T cell receptor (TCR) diversity of major alpha beta T cells in lymphoid tissues, epithelial T cells of the murine skin, called dendritic epidermal T cells (DETC), express exclusively an invariant gamma delta TCR. Fetal thymic precursors of DETC immigrate to the skin before birth, and in adult mice T cells expressing the canonical gamma delta TCR identical to that of DETC are not found in other lymphoid or epithelial tissues. Here, we show that DETC precursors migrate to the gut as well as to the skin during fetal periods, but preferentially survive and expand in the skin after birth. We propose that similar to the thymic selection of the diverse alpha beta T cell repertoire, 'peripheral selection' of the homogeneous epithelial gamma delta T cell repertoire may be mediated by TCR signaling upon the recognition of the self-ligand, because the ligand for the DETC TCR was expressed only in the skin.