Deborah A. Witherden

γδ T cell-induced hyaluronan production by epithelial cells regulates inflammation

Nonhealing wounds are a major complication of diseases such as diabetes and rheumatoid arthritis. For efficient tissue repair, inflammatory cells must infiltrate into the damaged tissue to orchestrate wound closure. Hyaluronan is involved in the inflammation associated with wound repair and binds the surface of leukocytes infiltrating damaged sites. Skin γδ T cells play specialized roles in keratinocyte proliferation during wound repair. Here, we show that γδ T cells are required for hyaluronan deposition in the extracellular matrix (ECM) and subsequent macrophage infiltration into wound sites. We describe a novel mechanism of control in which γδ T cell–derived keratinocyte growth factors induce epithelial cell production of hyaluronan. In turn, hyaluronan recruits macrophages to the site of damage. These results demonstrate a novel function for skin γδ T cells in inflammation and provide a new perspective on T cell regulation of ECM molecules.

The Junctional Adhesion Molecule JAML Is a Costimulatory Receptor for Epithelial γδ T Cell Activation

Skin Reaction Lymphocytes in the skin known as γδ T cells provide an important barrier against infection and injury. Unlike classical αβ T cells, less is known about the molecular requirements of γδ T cell activation. Two studies demonstrate that the junctional adhesion molecule–like protein (JAML) is a costimulatory molecule for mouse γδ T. Witherden et al. (p. 1205; see the Perspective by Shaw and Huang) showed that JAML, binding to its ligand Coxsackie and adenovirus receptor (CAR), leads to proliferation, as well as cytokine and growth factor production by γδ T cells. In vivo, JAML-CAR interactions contributed to the wound healing response in mice. Verdino et al. (p. 1210; see the Perspective by Shaw and Huang) present a crystal structure of CAR/JAML, which revealed an intracellular signaling motif similar to that known for the αβ T cell costimulatory receptor that signals through phosphoinositide 3-kinase. A costimulatory receptor for immune cells in the skin is identified. γδ T cells present in epithelial tissues provide a crucial first line of defense against environmental insults, including infection, trauma, and malignancy, yet the molecular events surrounding their activation remain poorly defined. Here we identify an epithelial γδ T cell–specific costimulatory molecule, junctional adhesion molecule–like protein (JAML). Binding of JAML to its ligand Coxsackie and adenovirus receptor (CAR) provides costimulation leading to cellular proliferation and cytokine and growth factor production. Inhibition of JAML costimulation leads to diminished γδ T cell activation and delayed wound closure akin to that seen in the absence of γδ T cells. Our results identify JAML as a crucial component of epithelial γδ T cell biology and have broader implications for CAR and JAML in tissue homeostasis and repair.

A Keratinocyte-Responsive γδ TCR Is Necessary for Dendritic Epidermal T Cell Activation by Damaged Keratinocytes and Maintenance in the Epidermis

A unique population of T lymphocytes, designated dendritic epidermal T cells (DETC), homes to the murine epidermis during fetal development. DETC express a canonical γδ TCR, Vγ3/Vδ1, which recognizes Ag expressed on damaged, stressed, or transformed keratinocytes. Recently, DETC were shown to play a key role in the complex process of wound repair. To examine the role of the DETC TCR in DETC localization to the epidermis, maintenance in the skin, and activation in vivo, we analyzed DETC in the TCRδ−/− mouse. Unlike previous reports in which the TCRδ−/− skin was found to be devoid of any DETC, we discovered that TCRδ−/− mice have αβ TCR-expressing DETC with a polyclonal Vβ chain repertoire. The αβ DETC are not retained over the life of the animal, suggesting that the γδ TCR is critical for the maintenance of DETC in the skin. Although the αβ DETC can be activated in response to direct stimulation, they do not respond to keratinocyte damage. Our results suggest that a keratinocyte-responsive TCR is necessary for DETC activation in response to keratinocyte damage and for DETC maintenance in the epidermis.

The molecular interaction of CAR and JAML recruits the central cell signal transducer PI3K.

Skin Reaction Lymphocytes in the skin known as γδ T cells provide an important barrier against infection and injury. Unlike classical αβ T cells, less is known about the molecular requirements of γδ T cell activation. Two studies demonstrate that the junctional adhesion molecule–like protein (JAML) is a costimulatory molecule for mouse γδ T. Witherden et al. (p. 1205; see the Perspective by Shaw and Huang) showed that JAML, binding to its ligand Coxsackie and adenovirus receptor (CAR), leads to proliferation, as well as cytokine and growth factor production by γδ T cells. In vivo, JAML-CAR interactions contributed to the wound healing response in mice. Verdino et al. (p. 1210; see the Perspective by Shaw and Huang) present a crystal structure of CAR/JAML, which revealed an intracellular signaling motif similar to that known for the αβ T cell costimulatory receptor that signals through phosphoinositide 3-kinase. Ligand engagement and initiation of signaling has been imaged for a costimulatory receptor for immune cells in the skin. Coxsackie and adenovirus receptor (CAR) is the primary cellular receptor for group B coxsackieviruses and most adenovirus serotypes and plays a crucial role in adenoviral gene therapy. Recent discovery of the interaction between junctional adhesion molecule–like protein (JAML) and CAR uncovered important functional roles in immunity, inflammation, and tissue homeostasis. Crystal structures of JAML ectodomain (2.2 angstroms) and its complex with CAR (2.8 angstroms) reveal an unusual immunoglobulin-domain assembly for JAML and a charged interface that confers high specificity. Biochemical and mutagenesis studies illustrate how CAR-mediated clustering of JAML recruits phosphoinositide 3-kinase (P13K) to a JAML intracellular sequence motif as delineated for the αβ T cell costimulatory receptor CD28. Thus, CAR and JAML are cell signaling receptors of the immune system with implications for asthma, cancer, and chronic nonhealing wounds.

The junctional adhesion molecule JAML is a costimulatory receptor for epithelial gammadelta T cell activation.

Skin Reaction Lymphocytes in the skin known as γδ T cells provide an important barrier against infection and injury. Unlike classical αβ T cells, less is known about the molecular requirements of γδ T cell activation. Two studies demonstrate that the junctional adhesion molecule–like protein (JAML) is a costimulatory molecule for mouse γδ T. Witherden et al. (p. 1205; see the Perspective by Shaw and Huang) showed that JAML, binding to its ligand Coxsackie and adenovirus receptor (CAR), leads to proliferation, as well as cytokine and growth factor production by γδ T cells. In vivo, JAML-CAR interactions contributed to the wound healing response in mice. Verdino et al. (p. 1210; see the Perspective by Shaw and Huang) present a crystal structure of CAR/JAML, which revealed an intracellular signaling motif similar to that known for the αβ T cell costimulatory receptor that signals through phosphoinositide 3-kinase. A costimulatory receptor for immune cells in the skin is identified. γδ T cells present in epithelial tissues provide a crucial first line of defense against environmental insults, including infection, trauma, and malignancy, yet the molecular events surrounding their activation remain poorly defined. Here we identify an epithelial γδ T cell–specific costimulatory molecule, junctional adhesion molecule–like protein (JAML). Binding of JAML to its ligand Coxsackie and adenovirus receptor (CAR) provides costimulation leading to cellular proliferation and cytokine and growth factor production. Inhibition of JAML costimulation leads to diminished γδ T cell activation and delayed wound closure akin to that seen in the absence of γδ T cells. Our results identify JAML as a crucial component of epithelial γδ T cell biology and have broader implications for CAR and JAML in tissue homeostasis and repair.

CD81 and CD28 Costimulate T Cells Through Distinct Pathways

We have examined the role of CD81 in the activation of murine splenic αβ T cells. Expression of the CD81 molecule on T cells increases following activation, raising the possibility of a role for this molecule in progression of the activation process. Using an in vitro costimulation assay, we show that CD81 can function as a costimulatory molecule on both CD4+ and CD8+ T cells. This costimulation functions independently of CD28, and unlike costimulation through CD28, is susceptible to inhibition by cyclosporin A. Strikingly, the pattern of cytokine production elicited by costimulation via CD81 is unique. IL-2 production was not up-regulated, whereas both IFN-γ and TNF-α expression significantly increased. Together our results demonstrate an alternate pathway for costimulation of T cell activation mediated by CD81.

γδ T Cell Homeostasis Is Controlled by IL-7 and IL-15 Together with Subset-Specific Factors

Among T cell subsets, γδ T cells uniquely display an Ag receptor-based tissue distribution, but what defines their preferential homing and homeostasis is unknown. To address this question, we studied the resources that control γδ T cell homeostasis in secondary lymphoid organs. We found that γδ and αβ T cells are controlled by partially overlapping resources, because acute homeostatic proliferation of γδ T cells was inhibited by an intact αβ T cell compartment, and both populations were dependent on IL-7 and IL-15. Significantly, to undergo acute homeostatic proliferation, γδ T cells also required their own depletion. Thus, γδ T cell homeostasis is maintained by trophic cytokines commonly used by other types of lymphoid cells, as well as by additional, as yet unidentified, γδ-specific factors.

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.

T-cell effector mechanisms: γδ and CD1d-restricted subsets

γδ T lymphocytes and CD1d-restricted natural killer T cells are classified as innate T lymphocytes, which perform effector functions that protect from malignancy and maintain tissue integrity. Innate T cells also play important regulatory roles in autoimmunity, inflammation and infection. Recent advances have established innate T cells as both effectors and regulators of disease in biological models.

Regulation of skin cell homeostasis by gamma delta T cells.

Although innate T lymphocytes such as gamma delta T cells have been extensively studied, their biological role has remained an enigma to researchers for many years. However, recent advances have begun to explain their complex role in the immune system. Gamma delta T cells are often the major T cell population in epithelial tissues such as the skin, gut, and lung where they have been implicated in maintaining tissue integrity, defending against pathogens, and regulating inflammation. The gamma delta T cells that reside in the skin are a prototypical intra-epithelial lymphocyte (IEL) population. These skin gamma delta T cell receptor (TCR)-expressing cells are named dendritic epidermal T cells (DETC) for their unique dendritic morphology. Using their gamma delta TCR, DETC recognize an unknown ligand expressed by stressed or damaged keratinocytes. Activated DETC exhibit effector functions such as cytokine production, cytolysis, and proliferation in vitro. Recent findings have shown that upon activation by damaged keratinocytes, DETC produce a key keratinocyte growth factor for wound repair called fibroblast growth factor 7 (FGF-7). FGF-7 is produced in vitro and in vivo, suggesting that DETC might play an important role in the biological function of wound repair. Indeed a delay in wound closure and a decrease in the proliferation of keratinocytes at the wound site have been observed in mice lacking gamma delta T cells. In addition to effector functions attributed to DETC, it has also been suggested that gamma delta T cells such as DETC have regulatory roles such as initiating or inhibiting inflammation. This is supported by the findings that DETC produce chemokines and cytokines. Control of the inflammatory response in the epithelium may provide another mechanism to reestablish homeostasis after a biological insult such as wound infliction. Understanding the function of DETC may be useful in the development of future therapies for chronic wounds and the maintenance of skin homeostasis.