A novel human psoriatic skin equivalent: the importance of transit amplifying keratinocytes

Psoriasis is an immune-mediated disease of the skin characterized by keratinocyte hyperproliferation, altered differentiation and resistance to apoptosis. Histologically, psoriasis is represented by augmented thickness of the epidermal compartment of the skin, acanthosis and intense immune cell infiltrate. Psoriasis is thought to be determined by both genetic and environmental components. Psoriasis has long been considered only an immunocyte-mediated disease, but recent data demonstrate the important role of keratinocytes in triggering the disease. Here we sought to develop an in vitro reconstructed skin model that would display the phenotypic and molecular characteristics of psoriatic epidermis in a controlled manner and in the absence of immune cells, providing a tool for the study of keratinocyte biology and allowing the screening of antipsoriatic drugs. Human skin equivalents were generated in the following way: the dermal compartment was a mixture of rat-tail collagen I and human fibroblasts either from healthy adult skin or from patients with psoriasis. Given the importance of transit amplifying (TA) cells in the pathogenesis of psoriasis, we used either TA cells or stem cells isolated from healthy human skin for the epidermal compartment. As a control, we also analysed skin equivalents raised from the total keratinocyte population. The combination of psoriatic fibroblasts and normal TA cells produced the best psoriatic phenotype, with a statistically significant increase of epidermal thickness, areas of acanthosis and expression of psoriatic markers, such as S100A7/psoriasin, K16 and phospho-Stat3 (Tyr705). Moreover, given the absence of p75NTR in psoriatic epidermis and the apoptotic role of p75NTR receptor mainly expressed by TA keratinocytes, we set up skin reconstructs with either p75NTR-positive or p75NTR-depleted TA cells. Skin reconstructs assembled with p75NTR-positive TA keratinocytes were almost negative for psoriatic markers. By contrast, skin reconstructs generated from p75NTR-depleted TA displayed a more psoriasiform phenotype. These results demonstrate the central role of TA cells in psoriasis together with the importance of psoriatic fibroblasts in triggering the psoriatic modifications.