Interleukin-17A+ Cell Counts Are Increased in Systemic Sclerosis Skin and Their Number Is Inversely Correlated with the Extent of Skin Involvement

Objective Levels of interleukin-17A (IL-17A) have been found to be increased in synovial fluid from individuals with systemic sclerosis (SSc). This study was undertaken to investigate whether IL-17A-producing cells are present in affected SSc skin, and whether IL-17A exerts a role in the transdifferentiation of myofibroblasts. Methods Skin biopsy samples were obtained from the involved skin of 8 SSc patients and from 8 healthy control donors undergoing plastic surgery. Immunohistochemistry and multicolor immunofluorescence techniques were used to identify and quantify the cell subsets in vivo, including IL-17A+, IL-4+, CD3+, tryptase-positive, α-smooth muscle actin (α-SMA)-positive, myeloperoxidase-positive, and CD1a+ cells. Dermal fibroblast cell lines were generated from all skin biopsy samples, and quantitative polymerase chain reaction, Western blotting, and solid-phase assays were used to quantify α-SMA, type I collagen, and matrix metalloproteinase 1 (MMP-1) production by the cultured fibroblasts. Results IL-17A+ cells were significantly more numerous in SSc skin than in healthy control skin (P = 0.0019) and were observed to be present in both the superficial and deep dermis. Involvement of both T cells and tryptase-positive mast cells in the production of IL-17A was observed. Fibroblasts positive for α-SMA were found adjacent to IL-17A+ cells, but not IL-4+ cells. However, IL-17A did not induce α-SMA expression in cultured fibroblasts. In the presence of IL-17A, the α-SMA expression induced in response to transforming growth factor β was decreased, while MMP-1 production was directly enhanced. Furthermore, the frequency of IL-17A+ cells was higher in the skin of SSc patients with greater severity of skin fibrosis (lower global skin thickness score). Conclusion IL-17A+ cells belonging to the innate and adaptive immune system are numerous in SSc skin. IL-17A participates in inflammation while exerting an inhibitory activity on myofibroblast transdifferentiation. These findings are consistent with the notion that IL-17A has a direct negative-regulatory role in the development of dermal fibrosis in humans.