Exposure of human skin to solar ultraviolet (UV) light induces local and systemic immune suppression. It is known that alterations of immune functions of Langerhans cells (LCs) and dermal dendritic cells (DDCs) mediate this phenomenon. The purpose of this study was to mimic in vitro the early UV-induced skin disruption to better understand the involvement of the skin micro-environment in triggering this immunosuppressive state. We therefore developed skin equivalents (SEs) integrating LCs and DDCs derived from monocytes (mo-LCs and mo-DDCs, respectively). First, we showed that Langerin(+) mo-LC and dendritic cell (DC)-specific ICAM-3 grabbing nonintegrin (SIGN)(+) mo-DDCs were immunolocalized in situ in epidermal and dermal compartments of SEs, respectively. The SE micro-environment without immune cells displayed full cytokine profile that may ensure and maintain differentiation, localization, and immaturity of LCs and DDCs in situ, as shown by secretion of granulocyte-macrophage colony-stimulating factor, transforming growth factor beta (beta)-1, interleukin (IL)-4, IL-13, and IL-15 involved in cell differentiation; presence of complete chemokine network as macrophage inflammatory protein 3 alpha (alpha); low secretion of pro-inflammatory cytokines tumor necrosis factor alpha (TNF-alpha), IL-1 beta, IL-6, and IL-8; and surprising secretion of immunosuppresive cytokine IL-10. Second, we demonstrated that skin micro-environment homeostasis was greatly disrupted under solar UV irradiation of SEs. In fact, we showed a pro-inflammatory state characterized by high secretion of TNF-alpha, IL-1 beta, IL-6, and IL-8 and low secretion of IL-10. This breakdown of immune homeostasis was visualized at the same time as in situ migration of mo-LCs and mo-DDCs into the dermal equivalent of SEs. Moreover, this tissue migration of mo-LCs and mo-DDCs into SEs was in accordance with the chemokine (C-C motif) receptor 7 expression and the DC-lysosome-associated membrane glycoprotein acquisition only on mo-LCs. Our results highlighted major participation of the skin micro-environment in the triggering and modulating of UV-induced skin immune responses. In addition, it could be concluded that these SEs are reliable tools for modeling biological events inaccessible in humans.
The events occurring during the penetration of melanoma cells through the dermal-epidermal junction, which is the first crucial step in the process of metastasis, are poorly understood, partly because no suitable tissue models exist. In the in vitro model reported here, two melanoma clones (T1C3, which generates lung metastases in experimental animals, and IC8, which does not) derived from a single parental cell line were co-seeded with normal allogenic keratinocytes onto acellular human de-epidermized dermis with preserved intact basement membrane and cultured for up to 1 month at an air-liquid interface. Histological, immunohistochemical and ultrastructural studies showed that melanoma cells from the metastatic clone (T1C3), but not from the non-metastatic clone (IC8), penetrated the dermal-epidermal junction to invade the dermis after 3 weeks of culture. Local invasion was associated with the dissolution of the native epidermal basement membrane collagens type IV and VII. Confocal laser scanning microscopy analysis demonstrated that numerous T1C3 cells were able to colonize the interstitial dermis and to rapidly penetrate empty dermal cavities. Our model represents a significant technical advance over others currently available since: (i) the organized three-dimensional architecture of the native dermal-epidermal junction is preserved; (ii) the active invasion process coincides with the dissolution of native components of the epidermal basement membrane, i.e. collagen types IV and VII; and (iii) the ability of melanoma cells to cross the dermal-epidermal junction correlates with their metastatic potential. This model provides a valuable tool for the study of the time-course of the cellular and molecular events that occur during the earliest steps of invasion in cutaneous melanoma. It also offers new opportunities to study the possible role of the keratinocyte environment in melanoma invasion.
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