Morphological evaluation of the effects of pro-inflammatory cytokines involved in the etiopathogenesis of atopic dermatitis in a 3D model of organotypic culture of normal human skin

Atopic Dermatitis (AD) is a chronic inflammatory skin disorder affecting nearly 20% of children, characterized by itch, frequent skin infections, and eczema. Skin barrier dysfunction is a critical component of AD, primarily triggered by interleukin (IL)-4 and IL-13. These pro-inflammatory cytokines affect keratinocyte terminal differentiation and tight junction (TJ) molecular composition. The consequent defective epidermal barrier facilitates environmental agent access, thus triggering i) inflammation, ii) induction of skin alarmins for keratinocyte activation, and iii) the adaptive immune system defense through Langerhans cells. The specific, direct and early keratinocyte response to either IL-4 or IL-13 is not well-documented yet, but a good possibility for the investigation is offered by the 3D model of organotypic culture of normal human skin standardized in our lab. Bioptic skin fragments obtained after aesthetic surgery of healthy women (n=6) were incubated for 24 h epidermal side-up in a Transwell system with either IL-4 or IL-13 or simple medium and routinely processed for immunofluorescence and transmission electron microscopy analysis. Indirect immunofluorescence analysis focused on the differentiation markers keratin (K)10 and K14. K16 and K17 were evaluated as skin injury biomarkers, whereas claudin-1 expression was investigated for TJs. IL-4 and IL-13 differently affected K10 expression in suprabasal layers, whilst K14 immunostaining was similar in all treated samples, slighter than in control group. Both cytokines induced K16 expression in spinous layers and no K17 positive keratinocytes were observed. Claudin-1 immunostaining was weaker with the exception of upper spinous and granular layers. IL-4 induced dilation of intercellular spaces and condensation of keratin filaments stronger than IL-13, in basal and lower spinous layers. In conclusion, in our experimental conditions, IL-4 and IL-13 specifically affected the distribution pattern of K10 and claudin-1. Moreover, the lack of K17 and the induction of K16 expression strongly suggest that this pro-inflammatory milieu is able to induce a peculiar cytoskeletal reorganization, which can be linked to AD.