UVB exposure of a humanized skin model reveals unexpected dynamic of keratinocyte proliferation and Wnt inhibitor balancing

We developed human dermo-epidermal skin substitutes which are presently applied in phase I and II clinical trials. Here we used these very same skin equivalents containing melanocytes, named MelSkin, as an experimental skin model. We investigated the effects of UVB irradiation on the skin grafts transplanted on immune-compromised rats. The irradiation induces a strong wound healing response going along with massive proliferation of basal keratinocytes, basically quiescent under non-irradiated, homeostatic conditions. As a consequence of UVB irradiation, the initially clearly defined basal keratinocyte (mono)layer expands into about three layers of keratinocytes, all of which still express the basal keratinocyte marker Keratin15. In contrast, epidermal melanocytes remain quiescent under these circumstances. Moreover, the Wnt inhibitors Dickkopf 3 and Wif1 are downregulated upon UVB irradiation in basal keratinocytes, whereas melanocytes continue to express Wnt inhibitors. These findings suggest that there is 1) a suprabasal population, proliferating in the homeostatic state, hence maintaining the integrity of the epidermis, and 2) a basal, usually quiescent keratinocyte population that is induced to massively proliferate upon irradiation. Importantly, the finding that MelSkin responds in a physiological fashion to UVB is of paramount importance in light of the planned clinical application