IL-25 in Atopic Dermatitis: A Possible Link between Inflammation and Skin Barrier Dysfunction?

Atopic dermatitis (AD) is a common skin disease associated with a TH2 response and increased levels of TH2-associated cytokines and IgE. The mechanisms resulting in skewing the immune response in a TH2 direction in AD are not fully elucidated. However, such skewing has recently been associated with IL-25 in a murine model for allergic airway disease. The aim of this study was to investigate whether IL-25 may have a role in AD. We have identified IL-25-producing cells within the dermis of AD patients and propose that these cells are dendritic cells (DCs). This is supported by in vitro experiments that indicate that monocyte-derived DCs are capable of producing IL-25. As null mutations of filaggrin are associated with the development of an impaired skin barrier in AD, we investigated whether IL-25 affects filaggrin synthesis by keratinocytes. Using mRNA analysis, we have shown that IL-25 stimulation does indeed decrease filaggrin synthesis in cultured keratinocytes. These results suggest that IL-25 produced by DCs could have a dual role as both an inducer of the TH2 response and as an inhibitor of filaggrin synthesis, thereby directly affecting skin barrier function in AD patients.