Background/Purpose: Systemic sclerosis (SSc) is a multisystem autoimmune disorder with clinical manifestations resulting from immune activation, vascular injuries and fibrosis development. Previous reports suggest that deficiency of the transcription factor Fli1 (Friend leukemia integration-1) has a pivotal role in the pathogenesis of SSc. Although Fli1 deficiency activates fibroblasts and endothelial cells toward an SSc phenotype in vitro, Fli1^+/- mice show no clinical symptoms similar to SSc, suggesting that some additional factors are required to develop SSc in those mice. To address this issue, we generated bleomycin (BLM)-induced SSc murine model using Fli1^+/-mice and evaluated their phenotype by focusing on fibroblasts, endothelial cells, and immune cells.

Methods: Wild type and Fli1^+/- mice were used in the BLM model of scleroderma. Degree of dermal thickness and fibrosis were determined by histological analyses. The quantity of the collagen-specific amino acid hydroxyproline was measured. Immunohistochemistry and real-time PCR were conducted to evaluate the degree of inflammation and the expression of cytokines, growth factors, chemokines, and cell adhesion molecules. The influence of Fli1 deficiency on the phenotypical changes in dermal fibroblasts, endothelial cells, and macrophages were also evaluated in vitro by real-time PCR and TGF-b bioassay.

Results: BLM induced dermal fibrosis to a much greater extent in Fli1^+/- mice than in wild type mice. In addition, upon BLM treatment, Fli1^+/- mice exhibited higher mRNA levels of CCN2 in the lesional skin than wild type mice, while mRNA levels of TGF-b were comparable. On the other hand, Fli1 haploinsufficiency greatly activated dermal fibroblasts in response to BLM treatment partly due to the elevated expression of integrin aVb3 and aVb5 in those cells, leading to the activation of latent TGF-b on their cell surface. Upon BLM treatment, the lesional skin of Fli1^+/- mice showed higher expression of IL-4, IL-6, IL-10, IFN-g, TNF-a, iNOS, arginase1, Fizz1, and Ym1 than that of wild type mice. The numbers of myofibroblasts, macrophages, and mast cells and the ratio of CD4⁺/CD8⁺ T cells were increased in the lesional skin of BLM-treated Fli1^+/- mice relative to that of BLM-treated wild type mice. Moreover, Fli1 haploinsufficiency promoted M2 macrophage infiltration in the lesional skin of BLM-treated mice and also promoted M2 differentiation of peritoneal macrophages by IL-4 or IL-13 stimulation in vitro. As for endothelial cells, Fli1 haploinsufficiency modulated the expression of cell adhesion molecules toward the induction of Th2 skewed inflammation by BLM treatment, as shown by the lower expression of E-selectin and P-selectin and the higher expression of ICAM-1 and GlyCAM-1 in the lesional skin of Fli1^+/-mice than in that of wild type mice.

Conclusion: With BLM treatment, Fli1^+/- mice exhibited exacerbated dermal fibrosis due to the phenotypical changes of fibroblasts, endothelial cells, and immune cells toward an SSc phenotype to a much greater extent than wild type mice. Some additional factors induced by BLM treatment may be required for Fli1^+/- mice to develop an SSc phenotype.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/fli1-haploinsufficiency-exacerbates-dermal-fibrosis-via-activation-of-fibroblasts-endothelial-cells-and-macrophages-in-bleomycin-treated-mice/