Body-barrier surveillance by epidermal γδ TCRs

The role of chemokines in cutaneous immunosurveillance

The skin serves as a critical barrier against pathogen entry. This protection is afforded by an array of skin-resident immune cells, which act as first-line responders against barrier breach and infection. The recruitment and positioning of these cells is controlled at multiple levels by endothelial cells, pericytes, perivascular macrophages and mast cells, and by the fibroblasts in the dermis and keratinocytes in the epidermis. Chemokine signalling through chemokine receptors expressed by the various leukocyte subsets is critical for their trafficking into and within the skin. The role of chemokines in the skin is complex, and remains incompletely understood despite three decades of investigation. Here, we review the roles that different chemokine pathways play in the skin, and highlight the recent developments in the field.

Epigenetic and transcriptional regulation of γδ T cell differentiation: Programming cells for responses in time and space

γδ T cells are major providers of the pro-inflammatory cytokines interferon-γ (IFNγ) and interleukin-17 (IL-17) in protective or pathogenic immune responses. Notably, murine γδ T cells commit to either IFNγ or IL-17 production during development in the thymus, before any subsequent activation in the periphery. Here we discuss the molecular networks that underlie thymic γδ T cell differentiation, as well as the mechanisms that sustain or modify their functional properties in the periphery. We concentrate on recent findings on lymphoid and tissue-resident γδ T cell subpopulations, with an emphasis on genome-wide studies and their added value to elucidate the regulation of γδ T cell differentiation at the transcriptional and epigenetic (chromatin) levels.

Dermal γδ T cells--What have we learned?

Over the last several years, a number of papers have called attention to a distinct population of γδ T cells preferentially found in the dermis of the skin of normal mice. These cells appear to play an important role in promoting the development of psoriasis, but also are critical for host resistance to particular pathogens. They are characterized by the expression of a limited subset of γδ T cell receptors and a strong propensity to secrete IL-17. Perhaps most importantly, humans appear to carry an equivalent dermal γδ T cell population, likewise biased to secrete IL-17 and also implicated as playing a pathogenic role in psoriasis. This review will attempt to summarize and reconcile recent findings concerning the dermal γδ T cells.

Direct and Indirect Effects of Cytomegalovirus-Induced γδ T Cells after Kidney Transplantation

Despite effective anti-viral therapies, cytomegalovirus (CMV) is still associated with direct (CMV disease) and indirect effects (rejection and poor graft survival) in kidney transplant recipients. Recently, an unconventional T cell population (collectively designated as Vδ2^neg γδ T cells) has been characterized during the anti-CMV immune response in all solid-organ and bone-marrow transplant recipients, neonates, and healthy people. These CMV-induced Vδ2^neg γδ T cells undergo a dramatic and stable expansion after CMV infection, in a conventional “adaptive” manner. Similarly, as CMV-specific CD8+ αβ T cells, they exhibit an effector/memory TEMRA phenotype and cytotoxic effector functions. Activation of Vδ2^neg γδ T cells by CMV-infected cells involves the γδ T cell receptor (TCR) and still ill-defined co-stimulatory molecules such as LFA-1. A multiple of Vδ2^neg γδ TCR ligands are apparently recognized on CMV-infected cells, the first one identified being the major histocompatibility complex-related molecule endothelial protein C receptor. A singularity of CMV-induced Vδ2^neg γδ T cells is to acquire CD16 expression and to exert an antibody-dependent cell-mediated inhibition on CMV replication, which is controlled by a specific cytokine microenvironment. Beyond the well-demonstrated direct anti-CMV effect of Vδ2^neg γδ T cells, unexpected indirect effects of these cells have been also observed in the context of kidney transplantation. CMV-induced Vδ2^neg γδ T cells have been involved in surveillance of malignancy subsequent to long-term immunosuppression. Moreover, CMV-induced CD16+ γδ T cells are cell effectors of antibody-mediated rejection of kidney transplants, and represent a new physiopathological contribution to the well-known association between CMV infection and poor graft survival. All these basic and clinical studies paved the road to the development of a future γδ T cell-based immunotherapy. In the meantime, γδ T cell monitoring should prove a valuable immunological biomarker in the management of CMV infection.

Force-dependent transition in the T-cell receptor β-subunit allosterically regulates peptide discrimination and pMHC bond lifetime

The αβ T-cell receptor (TCR) on each T lymphocyte mediates exquisite specificity for a particular foreign peptide bound to a major histocompatibility complex molecule (pMHC) displayed on the surface of altered cells. This recognition stimulates protection in the mammalian host against intracellular pathogens, including viruses, and involves piconewton forces that accompany pMHC ligation. Physical forces are generated by T-lymphocyte movement during immune surveillance as well as by cytoskeletal rearrangements at the immunological synapse following cessation of cell migration. The mechanistic explanation for how TCRs distinguish between foreign and self-peptides bound to a given MHC molecule is unclear: peptide residues themselves comprise few of the TCR contacts on the pMHC, and pathogen-derived peptides are scant among myriad self-peptides bound to the same MHC class arrayed on infected cells. Using optical tweezers and DNA tether spacer technology that permit piconewton force application and nanometer scale precision, we have determined how bioforces relate to self versus nonself discrimination. Single-molecule analyses involving isolated αβ-heterodimers as well as complete TCR complexes on T lymphocytes reveal that the FG loop in the β-subunit constant domain allosterically controls both the variable domain module's catch bond lifetime and peptide discrimination via force-driven conformational transition. In contrast to integrins, the TCR interrogates its ligand via a strong force-loaded state with release through a weakened, extended state. Our work defines a key element of TCR mechanotransduction, explaining why the FG loop structure evolved for adaptive immunity in αβ but not γδTCRs or immunoglobulins.

Clinical applications of gamma delta T cells with multivalent immunity

γδ T cells hold promise for adoptive immunotherapy because of their reactivity to bacteria, viruses, and tumors. However, these cells represent a small fraction (1–5%) of the peripheral T-cell pool and require activation and propagation to achieve clinical benefit. Aminobisphosphonates specifically expand the Vγ9Vδ2 subset of γδ T cells and have been used in clinical trials of cancer where objective responses were detected. The Vγ9Vδ2 T cell receptor (TCR) heterodimer binds multiple ligands and results in a multivalent attack by a monoclonal T cell population. Alternatively, populations of γδ T cells with oligoclonal or polyclonal TCR repertoire could be infused for broad-range specificity. However, this goal has been restricted by a lack of applicable expansion protocols for non-Vγ9Vδ2 cells. Recent advances using immobilized antigens, agonistic monoclonal antibodies (mAbs), tumor-derived artificial antigen presenting cells (aAPC), or combinations of activating mAbs and aAPC have been successful in expanding gamma delta T cells with oligoclonal or polyclonal TCR repertoires. Immobilized major histocompatibility complex Class-I chain-related A was a stimulus for γδ T cells expressing TCRδ1 isotypes, and plate-bound activating antibodies have expanded Vδ1 and Vδ2 cells ex vivo. Clinically sufficient quantities of TCRδ1, TCRδ2, and TCRδ1^negTCRδ2^neg have been produced following co-culture on aAPC, and these subsets displayed differences in memory phenotype and reactivity to tumors in vitro and in vivo. Gamma delta T cells are also amenable to genetic modification as evidenced by introduction of αβ TCRs, chimeric antigen receptors, and drug-resistance genes. This represents a promising future for the clinical application of oligoclonal or polyclonal γδ T cells in autologous and allogeneic settings that builds on current trials testing the safety and efficacy of Vγ9Vδ2 T cells.

γδ T cell surveillance via CD1 molecules

γδ T cells are a prominent epithelial-resident lymphocyte population, possessing multi-functional capacities in the repair of host tissue, pathogen clearance, and tumor surveillance. Although three decades have now passed since their discovery, the nature of γδ T cell receptor (TCR)-mediated ligand recognition remains poorly defined. Recent studies have provided structural insight into this recognition, demonstrating that γδ T cells survey both CD1 and the presented lipid, and in some cases are exquisitely lipid specific. We review these findings here, examining the molecular basis for and the functional relevance of this interaction. We discuss potential implications on the notion that non-classical major histocompatibility complex (MHC) molecules may function as important restricting elements of γδ TCR specificity, and on our understanding of γδ T cell activation and function.

Multiple Receptor-Ligand Interactions Direct Tissue-Resident γδ T Cell Activation

γδ T cells represent a major T cell population in epithelial tissues, such as skin, intestine, and lung, where they function in maintenance of the epithelium and provide a crucial first line defense against environmental and pathogenic insults. Despite their importance, the molecular mechanisms directing their activation and function have remained elusive. Epithelial-resident γδ T cells function through constant communication with neighboring cells, either via direct cell-to-cell contact or cell-to-matrix interactions. These intimate relationships allow γδ T cells to facilitate the maintenance of epithelial homeostasis, tissue repair following injury, inflammation, and protection from malignancy. Recent studies have identified a number of molecules involved in these complex interactions, under both homeostatic conditions, as well as following perturbation of these barrier tissues. These interactions are crucial to the timely production of cytokines, chemokines, growth factors, and extracellular matrix proteins for restoration of homeostasis. In this review, we discuss recent advances in understanding the mechanisms directing epithelial-T cell crosstalk and the distinct roles played by individual receptor-ligand pairs of cell surface molecules in this process.

Unconventional intraepithelial gut T cells: the TCR says it all

The intestinal epithelium harbors a large number of T cells, including TCRαβ cells that lack expression of CD4 and CD8αβ coreceptors. In this issue of Immunity, Mayans et al. (2014) and McDonald et al. (2014) shed light on the specificity and development of this enigmatic T cell population.

Expression of the T cell-specific adapter protein in human tissues

T cell-specific adapter protein (TSAd) encoded by the SH2D2A gene is expressed in activated T cells, NK cells and endothelial cells, but its tissue expression has not yet been mapped. Here, we have defined the specificity of two commercially available anti-TSAd monoclonal reagents using peptide arrays. We found them to bind separate epitopes in the C-terminal part of TSAd. We then used immunohistochemistry to examine TSAd expression in various human lymphoid and non-lymphoid tissues. Immunostaining of adjacent tissue sections revealed that a substantial fraction of CD3-positive cells in normal lymphoid and non-lymphoid tissues expressed TSAd. In particular, essentially all intra-epithelial T cells appeared to coexpress TSAd. In addition, TSAd expression was observed in endothelial cells of dermal microvessels, while it was not detected in endothelial cells of the other tested tissues. This work provides insight into the expression pattern of TSAd in various healthy human tissues.

Epithelial barrier function: at the front line of asthma immunology and allergic airway inflammation

Airway epithelial cells form a barrier to the outside world and are at the front line of mucosal immunity. Epithelial apical junctional complexes are multiprotein subunits that promote cell-cell adhesion and barrier integrity. Recent studies in the skin and gastrointestinal tract suggest that disruption of cell-cell junctions is required to initiate epithelial immune responses, but how this applies to mucosal immunity in the lung is not clear. Increasing evidence indicates that defective epithelial barrier function is a feature of airway inflammation in asthmatic patients. One challenge in this area is that barrier function and junctional integrity are difficult to study in the intact lung, but innovative approaches should provide new knowledge in this area in the near future. In this article we review the structure and function of epithelial apical junctional complexes, emphasizing how regulation of the epithelial barrier affects innate and adaptive immunity. We discuss why defective epithelial barrier function might be linked to TH2 polarization in asthmatic patients and propose a rheostat model of barrier dysfunction that implicates the size of inhaled allergen particles as an important factor influencing adaptive immunity.

Visualizing T Cell Migration in situ

Mounting a protective immune response is critically dependent on the orchestrated movement of cells within lymphoid tissues. The structure of secondary lymphoid organs regulates immune responses by promoting optimal cell-cell and cell-extracellular matrix interactions. Naïve T cells are initially activated by antigen presenting cells in secondary lymphoid organs. Following priming, effector T cells migrate to the site of infection to exert their functions. Majority of the effector cells die while a small population of antigen-specific T cells persists as memory cells in distinct anatomical locations. The persistence and location of memory cells in lymphoid and non-lymphoid tissues is critical to protect the host from re-infection. The localization of memory T cells is carefully regulated by several factors including the highly organized secondary lymphoid structure, the cellular expression of chemokine receptors and compartmentalized secretion of their cognate ligands. This balance between the anatomy and the ordered expression of cell surface and soluble proteins regulates the subtle choreography of T cell migration. In recent years, our understanding of cellular dynamics of T cells has been advanced by the development of new imaging techniques allowing in situ visualization of T cell responses. Here, we review the past and more recent studies that have utilized sophisticated imaging technologies to investigate the migration dynamics of naïve, effector, and memory T cells.

Tissue-resident T cells: dynamic players in skin immunity

The skin is a large and complex organ that acts as a critical barrier protecting the body from pathogens in the environment. Numerous heterogeneous populations of immune cells are found within skin, including some that remain resident and others that can enter and exit the skin as part of their migration program. Pathogen-specific CD8⁺ T cells that persist in the epidermis following infection are a unique population of memory cells with important roles in immune surveillance and protective responses to reinfection. How these tissue-resident memory T cells form in the skin, the signals controlling their persistence and behavior, and the mechanisms by which they mediate local recall responses are just beginning to be elucidated. Here, we discuss recent progress in understanding the roles of these skin-resident T cells and also highlight some of the key unanswered questions that need addressing.

Innate and Adaptive Immune Responses in Wound Epithelialization

Significance: Over the years, it has become clear that, in addition to performing their regular duties in immune defense, the innate and adaptive arms of the immune system are important regulators of the complex series of events that lead to wound healing. Immune cells modulate wound healing by promoting cellular cross-talk; they secrete signaling molecules, including cytokines, chemokines, and growth factors. In line with the major effort in wound healing research to find efficient therapeutic agents for the constantly increasing number of patients with chronic wounds, findings regarding the contributions of innate and adaptive immune responses to the re-epithelialization of damaged skin may bring novel therapeutics. Recent Advances: Increasing evidence suggests that induction of the adaptive immune response requires activation of innate immunity and that there is a dependent relationship between the two systems. Consequently, the bridge between the innate and the acquired immune systems has become an area of emerging exploration. It is clear that a better understanding of the epithelial cells (keratinocytes), immune cells, and mechanisms that contribute to an effective wound healing process is necessary so that new strategies for successful wounds treatment can be devised. Critical Issues: A greater understanding of the biology of skin innate and adaptive immune cells during wound epithelialization may have an impact on development of novel strategies for significant improvements in the quality of tissue repair. Future Directions: Future studies should clarify the importance of particular molecules and mechanisms utilized for development and functions of skin-resident γδT and Langerhans cells, as well as identify therapeutic targets for manipulation of these cells to combat epithelial diseases.

Orphan chemoattractant receptor GPR15 mediates dendritic epidermal T-cell recruitment to the skin

Homing of murine dendritic epidermal T cells (DETCs) from the thymus to the skin is regulated by specific trafficking receptors during late embryogenesis. Once in the epidermis, Vγ3δ1 TCR DETCs are maintained through self-renewal and participate in wound healing. GPR15 is an orphan G protein-linked chemoattractant receptor involved in the recruitment of regulatory T cells to the colon. Here we show that GPR15 is highly expressed on fetal thymic DETC precursors and on recently recruited DETCs, and mediates the earliest seeding of the epidermis, which occurs at the time of establishment of skin barrier function. DETCs in GPR15(-/-) mice remain low at birth, but later participation of CCR10 and CCR4 in DETC homing allows DETCs to reach near normal levels in adult skin. Our findings establish a role for GPR15 in skin lymphocyte homing and suggest that it may contribute to lymphocyte subset targeting to diverse epithelial sites.

Activation of the aryl hydrocarbon receptor dampens the severity of inflammatory skin conditions

Environmental stimuli are known to contribute to psoriasis pathogenesis and that of other autoimmune diseases, but the mechanisms are largely unknown. Here we show that the aryl hydrocarbon receptor (AhR), a transcription factor that senses environmental stimuli, modulates pathology in psoriasis. AhR-activating ligands reduced inflammation in the lesional skin of psoriasis patients, whereas AhR antagonists increased inflammation. Similarly, AhR signaling via the endogenous ligand FICZ reduced the inflammatory response in the imiquimod-induced model of skin inflammation and AhR-deficient mice exhibited a substantial exacerbation of the disease, compared to AhR-sufficient controls. Nonhematopoietic cells, in particular keratinocytes, were responsible for this hyperinflammatory response, which involved upregulation of AP-1 family members of transcription factors. Thus, our data suggest a critical role for AhR in the regulation of inflammatory responses and open the possibility for novel therapeutic strategies in chronic inflammatory disorders.

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Physiological AhR signals reduce psoriasis gene expression in patient biopsies

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Blocking AhR signals exacerbates psoriasis gene expression in patient biopsies

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AhR-deficient mice show exacerbated skin inflammation in imiquimod model

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Absence of AhR on mouse or human keratinocytes causes excessive inflammation

Persistence of skin-resident memory T cells within an epidermal niche

Barrier tissues such as the skin contain various populations of immune cells that contribute to protection from infections. These include recently identified tissue-resident memory T cells (TRM). In the skin, these memory CD8(+) T cells reside in the epidermis after being recruited to this site by infection or inflammation. In this study, we demonstrate prolonged persistence of epidermal TRM preferentially at the site of prior infection despite sustained migration. Computational simulation of TRM migration within the skin over long periods revealed that the slow rate of random migration effectively constrains these memory cells within the region of skin in which they form. Notably, formation of TRM involved a concomitant local reduction in dendritic epidermal γδ T-cell numbers in the epidermis, indicating that these populations persist in mutual exclusion and may compete for local survival signals. Accordingly, we show that expression of the aryl hydrocarbon receptor, a transcription factor important for dendritic epidermal γδ T-cell maintenance in skin, also contributes to the persistence of skin TRM. Together, these data suggest that skin tissue-resident memory T cells persist within a tightly regulated epidermal T-cell niche.

The multiple faces of leukocyte interstitial migration

Spatiotemporal control of leukocyte dynamics within tissues is critical for successful innate and adaptive immune responses. Homeostatic trafficking and coordinated infiltration into and within sites of inflammation and infection rely on signaling in response to extracellular cues that in turn controls a variety of intracellular protein networks regulating leukocyte motility, migration, chemotaxis, positioning, and cell-cell interaction. In contrast to mesenchymal cells, leukocytes migrate in an amoeboid fashion by rapid cycles of actin polymerization and actomyosin contraction, and their migration in tissues is generally referred to as low adhesive and nonproteolytic. The interplay of actin network expansion, contraction, and adhesion shapes the exact mode of amoeboid migration, and in this review, we explore how leukocyte subsets potentially harness the same basic biomechanical mechanisms in a cell-type-specific manner. Most of our detailed understanding of these processes derives from in vitro migration studies in three-dimensional gels and confined spaces that mimic geometrical aspects of physiological tissues. We summarize these in vitro results and then critically compare them to data from intravital imaging of leukocyte interstitial migration in mouse tissues. We outline the technical challenges of obtaining conclusive mechanistic results from intravital studies, discuss leukocyte migration strategies in vivo, and present examples of mode switching during physiological interstitial migration. These findings are also placed in the context of leukocyte migration defects in primary immunodeficiencies. This overview of both in vitro and in vivo studies highlights recent progress in understanding the molecular and biophysical mechanisms that shape robust leukocyte migration responses in physiologically complex and heterogeneous environments.

Tuning of antigen sensitivity by T cell receptor-dependent negative feedback controls T cell effector function in inflamed tissues

Activated T cells must mediate effector responses sufficiently to clear pathogens while avoiding excessive tissue damage. Here we have combined dynamic intravital microscopy with ex vivo assessments of T cell cytokine responses to generate a detailed spatiotemporal picture of CD4(+) T cell effector regulation in the skin. In response to antigen, effector T cells arrested transiently on antigen-presenting cells, briefly producing cytokine and then resuming migration. Antigen recognition led to upregulation of the programmed death-1 (PD-1) glycoprotein by T cells and blocking its canonical ligand, programmed death-ligand 1 (PD-L1), lengthened the duration of migration arrest and cytokine production, showing that PD-1 interaction with PD-L1 is a major negative feedback regulator of antigen responsiveness. We speculate that the immune system employs T cell recruitment, transient activation, and rapid desensitization to allow the T cell response to rapidly adjust to changes in antigen presentation and minimize collateral injury to the host.

Immune mediated shaping of microflora community composition depends on barrier site

Barrier surfaces, such as the intestinal lining and the skin, are colonized by a diverse community of commensal microorganisms. Although commensal microorganisms clearly impact the host immune system, whether the immune system also shapes the commensal community is poorly understood. We used 16S rDNA deep sequencing to test whether mice with specific immune defects have an altered commensal microflora. Initially, skin swabs were obtained from wild-type and Langerhans Cell (LC) deficient mice. Despite the intimate contacts that LC make with the upper epidermis, no significant differences were observed in microbial community composition. Similarly, the skin of MyD88/TRIF(-/-), Rag1(-/-) and heterozygous littermate controls showed no alteration in their commensal communities. Next we examined mouth swabs and feces. We did not find a difference in the MyD88/TRIF(-/-) mice. However, we did observe a significant shift in the microbial composition in the feces and mouths of Rag1(-/-) mice. Thus, we conclude that the adaptive immune system modulates the microbial composition at mucosal surfaces in the steady-state but LC, adaptive immunity, and MyD88-dependent innate responses do not affect the skin microbiome revealing a major distinction between barrier sites.

Innate-like T cells straddle innate and adaptive immunity by altering antigen-receptor responsiveness

The subclassification of immunology into innate and adaptive immunity is challenged by innate-like T lymphocytes that use innate receptors to respond rapidly to stress despite expressing T cell antigen receptors (TCRs), a hallmark of adaptive immunity. In studies that explain how such cells can straddle innate and adaptive immunity, we found that signaling via antigen receptors, whose conventional role is to facilitate clonal T cell activation, was critical for the development of innate-like T cells but then was rapidly attenuated, which accommodated the cells' innate responsiveness. These findings permitted the identification of a previously unknown innate-like T cell subset and indicate that T cell hyporesponsiveness, a state traditionally linked to tolerance, may be fundamental to T cells entering the innate compartment and thereby providing lymphoid stress surveillance.

The Skin-Resident Immune Network

The skin provides an effective physical and biological barrier against environmental and pathogenic insults whilst ensuring tolerance against commensal microbes. This protection is afforded by the unique anatomy and cellular composition of the skin, particularly the vast network of skin-associated immune cells. These include the long-appreciated tissue-resident macrophages, dendritic cells, and mast cells, as well as the more recently described dermal γδ T cells and innate lymphoid cells. Collectively, these cells orchestrate the defense against a wide range of pathogens and environmental challenges, but also perform a number of homeostatic functions. Here, we review recent developments in our understanding of the various roles that leukocyte subsets play in cutaneous immunobiology, and introduce the newer members of the skin immune system. Implications for human disease are discussed.

Dendritic epidermal T cells regulate skin antimicrobial barrier function

The epidermis, the outer layer of the skin, forms a physical and antimicrobial shield to protect the body from environmental threats. Skin injury severely compromises the epidermal barrier and requires immediate repair. Dendritic epidermal T cells (DETC) reside in the murine epidermis where they sense skin injury and serve as regulators and orchestrators of immune responses. Here, we determined that TCR stimulation and skin injury induces IL-17A production by a subset of DETC. This subset of IL-17A-producing DETC was distinct from IFN-γ producers, despite similar surface marker profiles. Functionally, blocking IL-17A or genetic deletion of IL-17A resulted in delayed wound closure in animals. Skin organ cultures from Tcrd-/-, which lack DETC, and Il17a-/- mice both exhibited wound-healing defects. Wound healing was fully restored by the addition of WT DETC, but only partially restored by IL-17A-deficient DETC, demonstrating the importance of IL-17A to wound healing. Following skin injury, DETC-derived IL-17A induced expression of multiple host-defense molecules in epidermal keratinocytes to promote healing. Together, these data provide a mechanistic link between IL-17A production by DETC, host-defense, and wound-healing responses in the skin. These findings establish a critical and unique role of IL-17A-producing DETC in epidermal barrier function and wound healing.

Lights, camera, and action: vertebrate skin sets the stage for immune cell interaction with arthropod-vectored pathogens

Despite increasing studies targeted at host-pathogen interactions, vector-borne diseases remain one of the largest economic health burdens worldwide. Such diseases are vectored by hematophagous arthropods that deposit pathogens into the vertebrate host's skin during a blood meal. These pathogens spend a substantial amount of time in the skin that allows for interaction with cutaneous immune cells, suggesting a window of opportunity for development of vaccine strategies. In particular, the recent availability of intravital imaging approaches has provided further insights into immune cell behavior in living tissues. Here, we discuss how such intravital imaging studies have contributed to our knowledge of cutaneous immune cell behavior and specifically, toward pathogen and tissue trauma from the arthropod bite. We also suggest future imaging approaches that may aid in better understanding of the complex interplay between arthropod-vectored pathogens and cutaneous immunity that could lead to improved therapeutic strategies.

The aryl hydrocarbon receptor in innate T cell immunity

Recent studies highlight an important role of the aryl hydrocarbon receptor (AhR) at mucosal barriers. Surprisingly, activation of the AhR, required for the maintenance of lymphocytes as well as lymphoid architecture, can be achieved via cues derived from the external environment. This environment contains both beneficial and harmful microorganisms as well as a diverse array of compounds, and the epithelia must offer very sophisticated levels of defence. This is achieved via multifaceted immune recognition diversity and cellular complexity. Mucosal associated tissues, particularly in the gastrointestinal tract, constitute a complex immune organ for local lymphocytes and contain highly organised lymphoid structures. We will discuss the recent observations concerning the AhR in relation to the function and maintenance of innate T cells, with focus on γδ T cells found enriched at epithelial barriers.

NKG2D triggers cytotoxicity in murine epidermal γδ T cells via PI3K-dependent, Syk/ZAP70-independent signaling pathway

Murine epidermal γδ T cells, known as dendritic epidermal T cells (DETCs), survey tissue stress through the invariant T-cell receptor (TCR) and non-clonotypic receptors such as NKG2D. NKG2D signaling via the DAP10-phosphatidylinositol 3-kinase (PI3K) pathway directly stimulates cytotoxicity in natural killer (NK) cells and costimulates CD8(+) T cells to augment TCR signals. In activated murine NK cells, NKG2D signals also via the DAP12-Syk/ZAP70 pathway that triggers both cytotoxicity and cytokine production. It remains controversial whether NKG2D on DETCs is a primary activating receptor or functions only as a costimulatory receptor, and signaling pathways initiated by NKG2D ligation in DETCs have not been analyzed. We show that stimulation of short-term DETC lines with recombinant NKG2D ligands triggers degranulation (exocytosis of cytotoxic granules) via the PI3K-dependent signaling pathway, but does not induce cytokine production or Syk/ZAP70 activation. Coengagement of TCR or Syk/ZAP70 signaling was not crucial for DETC-mediated killing of NKG2D ligand-expressing target cells. Thus, NKG2D can function as a coactivating stress receptor that directly triggers cytotoxicity in DETCs, at least after priming, via the PI3K-dependent, Syk/ZAP70-independent signaling pathway.

Effects of heat stress on the expression of the coxsackievirus and adenovirus receptor in mouse skin keratinocytes

The aim of this study was to investigate the effects of heat stress on the expression of the coxsackievirus and adenovirus receptor (CAR) in mouse skin keratinocytes. Twenty BALB/c mice were randomly divided into two groups: the sham heat (control) and scald groups. Skin specimens were obtained 6 h after the treatments. Changes in the expression of CAR in skin keratinocyte samples were detected by immunohistochemistry, quantitative polymerase chain reaction and western blotting. In an in vitro assay, mouse skin keratinocytes were cultured and randomly divided into two groups: the normal control and heat stress groups. Six hours subsequently, the changes in CAR expression in the two groups were estimated by flow cytometry to determine the differences between the two groups. Heat stress significantly increased the expression of CAR in the mouse skin keratinocytes (P<0.05). The upregulation of CAR in mouse keratinocytes in burn wounds may be beneficial for restoring healing in organisms.

Cross-talk between intraepithelial γδ T cells and epithelial cells

Intraepithelial γδ T cells play pivotal roles in homeostasis, tissue repair, inflammation, and protection from malignancy. In some tissues, γδ T cells are the only resident T cell population, whereas in others, they coexist with αβ T cells and other lymphocyte populations. γδ T cell function in the epithelium requires constant communication between cells in the form of cell-to-cell contacts and cell-to-matrix interactions. These interactions coordinate with the timely production of specific cytokines, chemokines, growth factors, and glycosaminoglycans, which have specialized effects on neighboring epithelial cells. Antigens that activate these T cells are not well-defined, and they do not express classic costimulatory or coreceptor molecules. As such, an understanding of the mechanisms used by epithelial γδ T cells to maintain homeostasis and facilitate wound repair has necessitated the identification of novel molecular interactions between γδ T cells and their neighboring epithelial cells.

The origin and fate of γδT cell subsets

Recent experiments indicate that in contrast to αβT cells, γδT cell effector functions are largely preprogrammed in the thymus during fetal life. However the thymus also exports juvenile γδT cells that can mature and be polarized in the periphery. How these developmental pathways are regulated and how much they contribute to the γδT cell effector pool is unclear. Here we discuss recent advances in the understanding of γδT cell subset development, with particular focus on IL-17-producing γδT cells and their beneficial and pathogenic roles in immunity.

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.

γδ T-cell conference 2012: close encounters for the fifth time

The fifth international γδ T-cell conference was held in Freiburg, Germany, from May 31 to June 2, 2012, bringing together approximately 170 investigators from all over the world. The scientific program covered topics such as thymic development and the mechanisms of ligand recognition and activation, the interaction of γδ T cells with other immune and non-immune cells and its implications for homeostasis, infection, tissue repair and autoimmunity, and the role of γδ T cells in malignancy and their potential for novel immunotherapies. Here we discuss a selection of the oral communications at the conference, and summarise exciting new findings in the field regarding the development, mode of antigen recognition, and responses to microorganisms, viruses and tumours by human and mouse γδ T cells.

Functional immunoimaging: the revolution continues

Ten years ago, in 2002, the introduction of dynamic in vivo imaging to immunologists set a new standard for studying immune responses. In particular, two-photon imaging has provided tremendous insights into immune cell dynamics in various contexts, including infection, cancer, transplantation and autoimmunity. Whereas initial studies were restricted to the migration of and interactions between immune cells, recent advances are bringing intravital imaging to a new level in which cell dynamics and function can be investigated simultaneously. These exciting developments further broaden the applications of immunoimaging and provide unprecedented opportunities to probe and decode immune cell communication in situ.

Six-of-the-best: unique contributions of γδ T cells to immunology

γδ T cells are a unique and conserved population of lymphocytes that have been the subject of a recent explosion of interest owing to their essential contributions to many types of immune response and immunopathology. But what does the integration of recent and long-established studies really tell us about these cells and their place in immunology? The time is ripe to consider the evidence for their unique and crucial functions. We conclude that whereas B cells and αβ T cells are commonly thought to contribute primarily to the antigen-specific effector and memory phases of immunity, γδ T cells are distinct in that they combine conventional adaptive features (inherent in their T cell receptors and pleiotropic effector functions) with rapid, innate-like responses that can place them in the initiation phase of immune reactions. This underpins a revised perspective on lymphocyte biology and the regulation of immunogenicity.

Effector T-cell responses in non-lymphoid tissues: insights from in vivo imaging

T-cell responses are initiated within secondary lymphoid organs, and effector T-cells are released into the circulation where they home to inflamed tissues and mediate protective immune responses. Within non-lymphoid tissues, the types of cellular interactions and the dynamics that lead to clearance of infections are still relatively poorly understood. Here I review how imaging of effector T-cells within tissues has contributed to our understanding of immune responses, and examine some of the remaining questions that may benefit from in vivo imaging to reveal the intricacies of how immune cells function. A detailed understanding of the dynamics of T-cell responses within non-lymphoid tissues is important for the rational design of targeted therapies that influence key steps in disease progression.

Human γδ T-cell responses in infection and immunotherapy: common mechanisms, common mediators?

Upon receiving the Nobel Prize in Physiology or Medicine in 1987, Susumu Tonegawa referred to the then recent discovery of the γδ T-cell receptor and stated that "while the function of the T cells bearing this receptor is currently unknown (…) these T cells may be involved in an entirely new aspect of immunity". [Tonegawa, S., Scand. J. Immunol. 1993. 38: 303-319]. Twenty-five years of intense research later this ambivalent view still holds true. Immunologists now appreciate that γδ T cells indeed represent a highly intriguing "new aspect of immunity" that is unique and distinct from conventional lymphocytes, yet even scientists in the field still struggle to understand the molecular basis of γδ T-cell responses, especially with respect to the enigmatic mode of antigen recognition. Here, we portray the peculiar responsiveness of human Vγ9/Vδ2 T cells to microorganisms, tumor cells and aminobisphosphonates, in an attempt to integrate the corresponding - and at times confusing - findings into a "theory of everything" that may help explain how such diverse stimuli result in similar γδ T-cell responses via the recognition of soluble low molecular weight phosphoantigens.

Tissue-resident memory CD8+ T cells continuously patrol skin epithelia to quickly recognize local antigen

Recent work has demonstrated that following the clearance of infection a stable population of memory T cells remains present in peripheral organs and contributes to the control of secondary infections. However, little is known about how tissue-resident memory T cells behave in situ and how they encounter newly infected target cells. Here we demonstrate that antigen-specific CD8(+) T cells that remain in skin following herpes simplex virus infection show a steady-state crawling behavior in between keratinocytes. Spatially explicit simulations of the migration of these tissue-resident memory T cells indicate that the migratory dendritic behavior of these cells allows the detection of antigen-expressing target cells in physiologically relevant time frames of minutes to hours. Furthermore, we provide direct evidence for the identification of rare antigen-expressing epithelial cells by skin-patrolling memory T cells in vivo. These data demonstrate the existence of skin patrol by memory T cells and reveal the value of this patrol in the rapid detection of renewed infections at a previously infected site.

Defining the nature of human γδ T cells: a biographical sketch of the highly empathetic

The elusive task of defining the character of γδ T cells has been an evolving process for immunologists since stumbling upon their existence during the molecular characterization of the α and β T cell receptor genes of their better understood brethren. Defying the categorical rules used to distinctly characterize lymphocytes as either innate or adaptive in nature, γδ T cells inhabit a hybrid world of their own. At opposing ends of the simplified spectrum of modes of antigen recognition used by lymphocytes, natural killer and αβ T cells are particularly well equipped to respond to the 'missing self' and the 'dangerous non-self', respectively. However, between these two reductive extremes, we are chronically faced with the challenge of making peace with the 'safe non-self' and dealing with the inevitable 'distressed self', and it is within this more complex realm γδ T cells excel thanks to their highly empathetic nature. This review gives an overview of the latest insights revealing the unfolding story of human γδ T cells, providing a biographical sketch of these unique lymphocytes in an attempt to capture the essence of their fundamental nature and events that influence their life trajectory. What hangs in their balance is their nuanced ability to differentiate the friends from the foe and the pathological from the benign to help us adapt swiftly and efficiently to life's many stresses.

Potential of human γδ T cells for immunotherapy of osteosarcoma

Recurrent or metastatic osteosarcomas remain a challenging malignancy to treat. Therefore, development and testing of novel therapeutic strategies to target these patients are needed. Adoptive cellular therapy strategies are being evaluated intensively as a novel therapeutic strategy for cancer. Unlike αβ T cells requiring antigen processing and MHC-restricted peptide displayed by antigen-presenting cells, γδ T cells exhibit the potent MHC-unrestricted lytic activity against various tumors in vitro and in vivo. The recent considerable success of γδ T cell-based immunotherapy in lung metastasis of renal cell carcinoma warrants further efforts to apply this treatment to other cancers including osteosarcoma, especially recurrent and metastatic osteosarcomas. In this review, we summarize the available evidence on γδ T cell-based immunotherapy for osteosarcoma that has been achieved to date. More importantly, we discuss potential strategies of the combination of expanded γδ T cells and bisphosphonates, and modification and expansion of αβ TCR modified γδ T cells for improving its efficacy for the treatment of osteosarcoma.

Cytomegalovirus and tumor stress surveillance by binding of a human γδ T cell antigen receptor to endothelial protein C receptor

T cells bearing γδ T cell antigen receptors (TCRs) function in lymphoid stress surveillance. However, the contribution of γδ TCRs to such responses is unclear. Here we found that the TCR of a human V(γ)4V(δ)5 clone directly bound endothelial protein C receptor (EPCR), which allowed γδ T cells to recognize both endothelial cells targeted by cytomegalovirus and epithelial tumors. EPCR is a major histocompatibility complex-like molecule that binds lipids analogously to the antigen-presenting molecule CD1d. However, the V(γ)4V(δ)5 TCR bound EPCR independently of lipids, in an antibody-like way. Moreover, the recognition of target cells by γδ T cells required a multimolecular stress signature composed of EPCR and costimulatory ligand(s). Our results demonstrate how a γδ TCR mediates recognition of broadly stressed human cells by engaging a stress-regulated self antigen.

The CD100 receptor interacts with its plexin B2 ligand to regulate epidermal γδ T cell function

γδ T cells respond rapidly to keratinocyte damage, providing essential contributions to the skin wound healing process. The molecular interactions regulating their response are unknown. Here, we identify a role for interaction of plexin B2 with the CD100 receptor in epithelial repair. In vitro blocking of plexin B2 or CD100 inhibited γδ T cell activation. Furthermore, CD100 deficiency in vivo resulted in delayed repair of cutaneous wounds due to a disrupted γδ T cell response to keratinocyte damage. Ligation of CD100 in γδ T cells induced cellular rounding via signals through ERK kinase and cofilin. Defects in this rounding process were evident in the absence of CD100-mediated signals, thereby providing a mechanistic explanation for the defective wound healing in CD100-deficient animals. The discovery of immune functions for plexin B2 and CD100 provides insight into the complex cell-cell interactions between epithelial resident γδ T cells and the neighboring cells they support.

Dynamic migration of γδ intraepithelial lymphocytes requires occludin

γδ intraepithelial lymphocytes (IELs) are located beneath or between adjacent intestinal epithelial cells and are thought to contribute to homeostasis and disease pathogenesis. Using in vivo microscopy to image jejunal mucosa of GFP γδ T-cell transgenic mice, we discovered that γδ IELs migrate actively within the intraepithelial compartment and into the lamina propria. As a result, each γδ IEL contacts multiple epithelial cells. Occludin is concentrated at sites of γδ IEL/epithelial interaction, where it forms a ring surrounding the γδ IEL. In vitro analyses showed that occludin is expressed by epithelial and γδ T cells and that occludin derived from both cell types contributes to these rings and to γδ IEL migration within epithelial monolayers. In vivo TNF administration, which results in epithelial occludin endocytosis, reduces γδ IEL migration. Further in vivo analyses demonstrated that occludin KO γδ T cells are defective in both initial accumulation and migration within the intraepithelial compartment. These data challenge the paradigm that γδ IELs are stationary in the intestinal epithelium and demonstrate that γδ IELs migrate dynamically to make extensive contacts with epithelial cells. The identification of occludin as an essential factor in γδ IEL migration provides insight into the molecular regulation of γδ IEL/epithelial interactions.