Julie M. Jameson

Dendritic epidermal T cells regulate skin homeostasis through local production of insulin-like growth factor 1

A fine balance between rates of proliferation and apoptosis in the skin provides a defensive barrier and a mechanism for tissue repair after damage. Vγ3+ dendritic epidermal T cells (DETCs) are primary modulators of skin immune responses. Here we show that DETCs both produce and respond to insulin-like growth factor 1 (IGF-1) after T cell receptor stimulation. Mice deficient in DETCs had a notable increase in epidermal apoptosis that was abrogated by the addition of DETCs or IGF-1. Furthermore, DETC-deficient mice had reduced IGF-1 receptor activation at wound sites. These findings indicate critical functions for DETC-mediated IGF-1 production in regulating skin homeostasis and repair.

A role for human skin–resident T cells in wound healing

Epidermal T cells have been shown to play unique roles in tissue homeostasis and repair in mice through local secretion of distinct growth factors in the skin. Human epidermis contains both αβ+ and γδ+ T cells whose functional capabilities are not understood. We demonstrate that human epidermal T cells are able to produce insulin-like growth factor 1 (IGF-1) upon activation and promote wound healing in a skin organ culture model. Moreover, an analysis of the functional capabilities of T cells isolated from acute versus chronic wounds revealed a striking difference. Both αβ+ and Vδ1+ T cells isolated from acute wounds actively produced IGF-1, demonstrating that they are activated during tissue damage to participate in wound repair. In contrast, IGF-1 production could not be detected in T cells isolated from chronic wounds. In fact, skin T cells isolated from chronic wounds were refractory to further stimulation, suggesting an unresponsive state. Collectively, these results define a novel role for human epidermis–resident T cells in wound healing and provide new insight into our understanding of chronic wound persistence.

Skin γδ T‐cell functions in homeostasis and wound healing

Summary:  There is a resident population of T cells found in murine skin that expresses an invariant Vγ3Vδ1 T‐cell receptor (TCR), and these cells are significantly different from lymphoid γδ T cells and αβ T cells in terms of ontogeny, tissue tropism, and antigen receptor diversity. These dendritic epidermal T cells are derived from fetal thymic precursor cells, are in constant contact with neighboring epidermal cells, and express a monoclonal γδTCR only found in the skin. Skin γδ T cells have been shown to play unique roles in tissue homeostasis and during tissue repair through local secretion of distinct growth factors including keratinocyte growth factors and insulin‐like growth factor‐1. In this review, we discuss evidence supporting a role for cross talk between skin γδ T cells and keratinocytes that contributes to the maintenance of normal skin and wound healing.

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.

Epidermal T Cells and Wound Healing

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

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.

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.

Cutting Edge: Dendritic Epidermal γδ T Cell Ligands Are Rapidly and Locally Expressed by Keratinocytes following Cutaneous Wounding

TCR-specific activation is pivotal to dendritic epidermal T cell (DETC) function during cutaneous wound repair. However, DETC TCR ligands are uncharacterized, and little is known about their expression patterns and kinetics. Using soluble DETC TCR tetramers, we demonstrate that DETC TCR ligands are not constitutively expressed in healthy tissue but are rapidly upregulated following wounding on keratinocytes bordering wound edges. Ligand expression is tightly regulated, with downmodulation following DETC activation. Early inhibition of TCR–ligand interactions using DETC TCR tetramers delays wound repair in vivo, highlighting DETC as rapid responders to injury. To our knowledge, this is the first visualization of DETC TCR ligand expression, which provides novel information about how ligand expression impacts early stages of DETC activation and wound repair.

Defects in Skin γδ T Cell Function Contribute to Delayed Wound Repair in Rapamycin-Treated Mice

Disruptions in the normal program of tissue repair can result in poor wound healing, which perturbs the integrity of barrier tissues such as the skin. Such defects in wound repair occur in transplant recipients treated with the immunosuppressant drug rapamycin (sirolimus). Intraepithelial lymphocytes, such as γδ T cells in the skin, mediate tissue repair through the production of cytokines and growth factors. The capacity of skin-resident T cells to function during rapamycin treatment was analyzed in a mouse model of wound repair. Rapamycin treatment renders skin γδ T cells unable to proliferate, migrate, and produce normal levels of growth factors. The observed impairment of skin γδ T cell function is directly related to the inhibitory action of rapamycin on mammalian target of rapamycin. Skin γδ T cells treated with rapamycin are refractory to IL-2 stimulation and attempt to survive in the absence of cytokine and growth factor signaling by undergoing autophagy. Normal wound closure can be restored in rapamycin-treated mice by addition of the skin γδ T cell-produced factor, insulin-like growth factor-1. These studies not only reveal that mammalian target of rapamycin is a master regulator of γδ T cell function but also provide a novel mechanism for the increased susceptibility to nonhealing wounds that occurs during rapamycin administration.