IL-25 participates in keratinocyte-driven dermal matrix turnover and is reduced in Systemic Sclerosis epidermis

Aberrant deposition of extracellular matrix (ECM) resulting in dermal fibrosis is a hallmark of systemic sclerosis (SSc). Evidence suggests that dysfunctional SSc keratinocytes may contribute to fibrosis by altering dermal homeostasis. Whether interleukin-25 (IL-25), an IL-17 family member involved in epithelial/mesenchymal/immune cell interplay takes part in skin fibrosis is unknown. Here we address the role of IL-25 in SSc skin fibrosis. Compared to healthy donor (HD), in SSc and scleroderma-like disorders the epidermis expressed significantly lower levels of IL-25. In epidermal equivalents, IL-25 regulated several molecular pathways related to wound healing and ECM remodeling. Compared to control conditioned medium (CM), the CM from IL-25-primed keratinocytes enhanced the production by fibroblasts of matrix metalloproteinase-1 (MMP-1), IL-6, IL-8 (p< 0.05), but not of type-I collagen (Col-I ) nor fibronectin. However, IL- 25 significantly reduced the production of Col-I when applied directly to fibroblasts. The activation of keratinocytes by IL-25 was receptor-dependent and evident after a very short incubation time (10 min), largely mediated by IL-1, suggesting enhanced and specific release of preformed mediators. These results show that IL-25 participates to skin homeostasis and its decreased expression in SSc may contribute to skin fibrosis by favoring ECM deposition over degradation. Overall design: mRNA profile of IL-25 treated epidermal equivalent reconstituted from 3 different primary human adult keratinocytes were generated by deep sequencing, in triplicate, using Illumina HiSeq 4000 sequencer.