Regulatory T cells in skin injury: At the crossroads of tolerance and tissue repair

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Abstract

Skin injury is a highly inflammatory process that is carefully regulated to mitigate tissue damage and allow for proper barrier repair. Regulatory T cells (T _regs) are crucial coordinators of the immune response to injury in several organs. Here, we review the emerging role of T _regs in facilitating skin repair after injury. We focus on recently discovered interactions between lymphocytes and nonhematopoietic cells during wound healing and discuss how these interactions are regulated both by “classical” suppressive mechanisms of T _regs and by “nonclassical” reparative T _reg functions.

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Distinct fibroblast lineages determine dermal architecture in skin development and repair

Ryan R. Driskell, Beate M Lichtenberger, Esther Hoste … (2013)

Fibroblasts are the major mesenchymal cell type in connective tissue and deposit the collagen and elastic fibers of the extracellular matrix (ECM) 1 . Even within a single tissue fibroblasts exhibit remarkable functional diversity, but it is not known whether this reflects the existence of a differentiation hierarchy or is a response to different environmental factors. Here we show, using transplantation assays and lineage tracing, that the fibroblasts of skin connective tissue arise from two distinct lineages. One forms the upper dermis, including the dermal papilla that regulates hair growth and the arrector pili muscle (APM), which controls piloerection. The other forms the lower dermis, including the reticular fibroblasts that synthesise the bulk of the fibrillar ECM, and the pre-adipocytes and adipocytes of the hypodermis. The upper lineage is required for hair follicle formation. In wounded adult skin, the initial wave of dermal repair is mediated by the lower lineage and upper dermal fibroblasts are recruited only during re-epithelialisation. Epidermal beta-catenin activation stimulates expansion of the upper dermal lineage, rendering wounds permissive for hair follicle formation. Our findings explain why wounding is linked to formation of ECM-rich scar tissue that lacks hair follicles 2-4 . They also form a platform for discovering fibroblast lineages in other tissues and for examining fibroblast changes in ageing and disease.

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Type 2 innate lymphoid cells control eosinophil homeostasis

Jesse C. Nussbaum, Steven Van Dyken, Jakob von Moltke … (2013)

Eosinophils are specialized myeloid cells associated with allergy and helminth infections. Blood eosinophils demonstrate circadian cycling, as described over 80 years ago, 1 and are abundant in the healthy gastrointestinal tract. Although a cytokine, interleukin (IL)-5, and chemokines such as eotaxins, mediate eosinophil development and survival, 2 and tissue recruitment, 3 respectively, the processes underlying the basal regulation of these signals remain unknown. Here, we show that serum IL-5 is maintained by long-lived type 2 innate lymphoid cells (ILC2) resident in peripheral tissues. ILC2 secrete IL-5 constitutively and are induced to co-express IL-13 during type 2 inflammation, resulting in localized eotaxin production and eosinophil accumulation. In the small intestine where eosinophils and eotaxin are constitutive, 4 ILC2 co-express IL-5 and IL-13, which is enhanced after caloric intake. The circadian synchronizer vasoactive intestinal peptide (VIP) also stimulates ILC2 through the VPAC2 receptor to release IL-5, linking eosinophil levels with metabolic cycling. Tissue ILC2 regulate basal eosinophilopoiesis and tissue eosinophil accumulation through constitutive and stimulated cytokine expression, and this dissociated regulation can be tuned by nutrient intake and central circadian rhythms.

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T-bet controls regulatory T cell homeostasis and function during type-1 inflammation

Meghan Koch, Glady’s Tucker-Heard, Nikole R. Perdue … (2009)

Several subsets of Foxp3+ regulatory T (Treg) cells work in concert to maintain immune homeostasis. However, the molecular bases underlying the phenotypic and functional diversity of Treg cells remain obscure. We show that in response to interferon-γ, Foxp3+ Treg cells upregulated the T helper 1 (TH1)-specifying transcription factor T-bet. T-bet promoted expression of the chemokine receptor CXCR3 on Treg cells, and T-bet+ Treg cells accumulated at sites of TH1-mediated inflammation. Furthermore, T-bet expression was required for the homeostasis and function of Treg cells during type-1 inflammation. Thus, within a subset of CD4+ T cells, the activities of Foxp3 and T-bet are overlaid, resulting in Treg cells with unique homeostatic and migratory properties optimized for suppression of TH1 responses in vivo.