Previous clinical and experimental studies on the role of DCs in SSc development have been predominantly focused on pDCs that contribute to sterile inflammation related to autoimmune rheumatic disorders through type I interferon (IFN-I) production in response to self-nucleic acids . Indeed, the IFN signature has been observed in the peripheral blood, skin and lungs of SSc patients [32, 33], and circulating SSc pDCs secrete elevated levels of IFN-I due to the aberrantly expressed Toll-like receptor (TLR) 8 as well as TLR7 and TLR9. Importantly, pDC depletion not only prevents the development of skin fibrosis, but also ameliorates established skin fibrosis in BLM-treated mice . Thus, it is currently accepted that pDCs play a critical role in SSc pathogenesis. On the other hand, although cDCs act as a critical regulator of skin immunity , their roles in immunopathology of SSc have remained poorly understood.
In this study, we employed Fli1+/− mice to investigate the phenotypical alteration of cDCs with relevance to SSc-associated disease pathology because BLM-treated Fli1+/− mice exhibit SSc-like features in various cell types [28, 35]. Since BLM injection induces RALDH1 production in CD103−CD11b− dermal cDCs and subsequently promotes Treg development in WT mice , the enhancement of BLM-dependent dermal fibrosis in Fli1+/− mice suggests that Fli1 deficiency impairs the regulatory function of CD103−CD11b− dermal cDCs. As expected, RALDH activity was decreased in CD103−CD11b− dermal cDCs of BLM-treated Fli1+/− mice. Furthermore, the Treg proportion was decreased in the dermis of BLM-treated Fli1+/− mice. Although we could not exclude the possibility that Fli1 deficiency directly affects the differentiation of naïve CD4+ T cells to Tregs, these current results indicate that decreased RALDH1 activity of CD103−CD11b− DCs at least partially contributes to the dysregulated Treg induction in the skin of BLM-treated Fli1+/− mice. This notion is supported by our previous finding that Raldh1 siRNA suppresses Treg induction in the skin of BLM-treated WT mice . Considering decreased expression of Fli1 and RALDH1 in CD11c+ cells as shown in our previous and current studies and exclusive expression of RALDH in cDCs [9, 11], we can plausibly conclude that Fli1 deficiency induces SSc-like phenotypes in dermal DCs, in addition to previously shown fibroblasts, endothelial cells, macrophages and epithelial cells [20, 28, 29, 31, 36, 37].
The alteration of T cell subsets has been well studied in the field of SSc research. Generally, the balances of Th1/Th2 and Th17/Treg immune responses skew toward Th2 and Th17 predominance [38–41], and Treg function is impaired in the active stage of SSc . In addition, the proportion of Th2-like Tregs is increased in the involved skin of SSc patients . Importantly, these characteristic alterations of CD4+ T cell balance are reproduced in BLM-treated Fli1+/− mice , and some of the potential underlying mechanisms have been shown. For instance, Fli1 deficiency induces the expression of IL-33 and galectin-9 in dermal fibroblasts, resulting in the promotion of Th2-like Treg induction and the suppression of Th1 development, respectively [28, 35]. Also, Fli1 deficiency facilitates IL-6 production in dermal fibroblasts, promoting Th17 differentiation together with active TGF-β and IL-1β that are elevated in the skin of BLM-treated Fli1+/− mice . The current study added a new insight into the contribution of Fli1 deficiency to the induction of SSc-like CD4+ T cell balance; namely, the decrease in Treg proportion due to the reduced RALDH1 expression in CD103−CD11b− dermal cDCs. Thus, Fli1 deficiency may serve as a key regulator of skin inflammation, as well as skin fibrosis and vasculopathy, in SSc.
The current study suggests that administration of retinoic acid is a potential therapeutic strategy to increase the proportion of Tregs in SSc . However, it has been long recognized that overexposure to retinoic acid causes widespread teratogenesis in humans . An alternative therapeutic strategy is to restore Fli1 expression in cDCs, possibly resulting in increased RALDH1 activity of cDCs and subsequent induction of Tregs in the involved skin of SSc patients. From this perspective, it is quite important to identify the mechanism by which Fli1 is downregulated in SSc cDCs. In dermal fibroblasts and dermal microvascular endothelial cells, endothelin-1 stimulation suppresses Fli1 expression and induces SSc-like phenotypes [36, 37, 46]. Importantly, endothelin-1 (ET-1) directly induces the phenotypic maturation of bone marrow-derived DCs that preferentially prime T cells to produce Th17 cytokines . Given that RALDH1-producing cDCs skew Th17/Treg balance toward Treg predominance [9, 44], ET-1 may be a possible factor suppressing RALDH1 production in cDCs, in which ET-1-dependent Fli1 suppression may be involved. Since bosentan, a dual antagonist of endothelin receptors, reverses SSc-like phenotypes of dermal microvascular endothelial cells by increasing Fli1 expression and improves SSc vascular features at the molecular and morphological levels [30, 36, 37, 48–50], this reagent may modulate skin immunity of SSc patients. Further studies are required to clarify this point.