Fli1 deficiency suppresses RALDH1 activity of dermal dendritic cells and related induction of regulatory T cells: a possible role in scleroderma

Background: Aldehyde dehydrogenase 1 family member A1 (RALDH1)-producing dermal dendritic cells (DCs), a conventional DC subset regulating skin fibrosis, are decreased in the involved skin of patients with systemic sclerosis (SSc). In this study, we investigated the contribution of Fli1 deficiency, a potential predisposing factor of SSc, to the phenotypical alteration of RALDH1-producing dermal DCs by using SSc model mice and SSc skin samples. Methods: Bleomycin (BLM)-induced skin fibrosis was generated with Fli1^+/−and wild-type mice. The proportions of DC and CD4⁺T cell subsets were determined by flow cytometry in the dermis of BLM-treated mice. Fli1 expression in dermal DCs was evaluated by immunofluorescence with skin samples of SSc and healthy control subjects. Results: RALDH activity of dermal DCs was significantly decreased in BLM-treated Fli1^+/−mice compared with BLM-treated wild-type mice, whereas the proportion of CD103⁻CD11b⁻dermal DCs, a major DC subset producing RALDH1 in response to BLM injection, was comparable between groups. Relevant to this finding, the proportion of regulatory T cells (Tregs) in the dermis was decreased in BLM-treated Fli1^+/−mice relative to BLM-treated wild-type mice, while the proportions of Th1, Th2 and Th17 cells were unaltered. In the involved skin of SSc patients, Fli1 was downregulated in CD11c⁺cells, including dermal DCs. Conclusions: Fli1 deficiency inhibits RALDH1 activity of CD103⁻CD11b⁻dermal DCs and related induction of Tregs in BLM-treated mice. Considering Fli1 reduction in SSc dermal DCs, Fli1deficiency may impair the dermal DC-Treg system, contributing to the development of skin fibrosis in SSc.