Disruption of skin natural tolerance by UV irradiation

ARTICLE

Disruption of skin natural tolerance by UV irradiation

Auteur(s) : Nicolas BECHETOILLE¹, Colette DEZUTTER-DAMBUYANT³, Valérie ANDRɹ, Odile DAMOUR², Eric PERRIER¹

¹ ENGELHARD, 32 rue Saint Jean de Dieu, 69007 Lyon, France
² Laboratoire des Substituts Cutanés, Hôpital Ed. Herriot, Lyon, France
³ Equipe d’Accueil Universitaire 32-37, Université Claude Bernard Lyon 1, France

We describe an immunologically stable skin equivalent (SE) in which UV-induced deregulation of cutaneous immunological homeostasis and the development of specific immunity can be studied simultaneously.
Cutaneous dendritic cells (DC), Langerhans cells (LC) and dermal DC (DDC), interact closely with their micro-environment in order to maintain homeostasis of the skin and develop appropriate immune responses. In UV irradiated skin, the immune functions of LC and DDC are greatly changed, leading, in the long term, to phenomena of immuno-suppression and peripheral tolerance [1]. Here, we report for the first time the effects of photo-induced cutaneous deregulation associated with the disruption of skin homeostasis and the specific targeting of LC and DDC. A lack of tools other than the mouse model is a drawback for the study of cutaneous homeostasis and its photo-induced deregulation in man. The ex vivo culture of an explant of normal human skin is not a satisfactory model, due to its pseudo-inflammatory character that induces, among other effects, the spontaneous activation and migration of LC [2]. For this reason, we have developed a human skin equivalent whose cell physiology and matrix organization are very close to normal human skin at rest [3]. In this context, SEs do not present a constitutive and/or spontaneous inflammatory phenotype as shown by a very low basal secretion of the main pro-inflammatory cytokines of the skin. When LC and DDC (derived from monocytes) are integrated into SEs, they express their characteristic markers and are localized exclusively in the epidermal and dermal compartments of the cultures, respectively (figure 1A). Fortunately, the LC and DDC do not migrate, auto-activate (CCR7 ^– ) or mature (DC-LAMP ^– ) spontaneously in the SE in contrast to explant skin cultures and freshly-isolated human LC [4]. Hence our SE model has the advantage of maintaining the LC and DDC in a state of differentiation and immaturity close to their in vivo homologues, which is essential for studying the regulatory phenomena of skin homeostasis and the specific targeting of the two cutaneous DC populations.
We then irradiated the SE containing LC and DDC with UV radiation (526 kJ/m² of solar energy or 2 J/cm² of UVA and 0.5 J/cm² of UVB). As observed in vivo after exposure to the sun, LC and DDC lost their dendricity and acquired the morphology of migratory cells with a rounded cellular body. In this context, a significant number of Langerin⁺ LC were observed in the dermal compartment of the SE, in agreement with the absence/great reduction of cells in the epidermal compartment (figure 1B). Concerning the DC-SIGN⁺ DDC, their localization in the deep layers of the dermal equivalent also suggested their migration (figure 1C). Interestingly, only the Langerin⁺ LC migrating in the dermal equivalent acquired the expression of the DC-LAMP maturation marker after solar UV irradiation (figures 1B and 1C). In other words, UV appears to have immuno-activating effects on LC, as has been observed only in vivo [5] but not in vitro or ex vivo [6]. Furthermore, the differential UV-induced responses of LC and DDC raise the question of the potential existence of a population of immunoactivating cutaneous DC. This light-induced targeting of LC and DDC seems to be the result of disrupting homeostasis in the skin. Indeed, the cutaneous tissue environment (keratinocytes + fibroblasts) secretes high quantities of IL-10 (figure 1D), which is probably responsible for the immature state of the LC and DDC in SE. However, after solar UV irradiation, secretion of this immunosuppressive cytokine drops sharply (figure 1D) leading to an imbalance in cutaneous tolerance, resulting in the activation and migration of DDC and the phenotypic maturation of LC.
The development of tools for modeling ex-vivo cutaneous physiology close to that of normal human skin is now essential to link the UV-induced premature mechanisms of disregulation of skin homeostasis with disruption of natural tolerance and the development of cutaneous immunity. n

References

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