*Purpose: This study was designed to investigate the effect and mechanisms of Iguratimod on circulating DSA and allograft survival in a skin transplant model.

*Methods: Mice secondary skin transplantation model was established using the C57BL/6 and Balb/c as the donor and recipients, respectively. Iguratimod (35mg/kg) were administrated in the first/secondary skin transplanted recipients by intragastric administration. Levels of de novo DSAs following the first skin transplant and secondary DSAs after second skin transplant in each group were detected by flow cytometry assay. Levels of Bregs (CD19+Tim-1+), Tregs (CD4+CD25+ Foxp3+), as well as the Th17 cells (CD4+IL-17A+), in peripheral blood monocytes (PBMCs) collected from mice recipients were also examined. Various inflammatory cytokines, such as IL-2, IL-4, IL-10, and IL-17, were tested by ELISA assay. To further explore the molecular mechanisms, primary Bregs extracted from the spleen of recipients with or without Iguratimod (10μg/ml), as well as PBMCs, were co-cultured in vitro. De novo DSAs were detected. Moreover, the Bregs and balance of Th17/Tregs were examined. Finally, the co-culture system was intervened by anti-IL-17 receptor (IL-17R) monoclonal antibody to explore the down-stream mechanism. Significantly reduction of DSAs was observed after the treatment with Iguratimod in secondary skin transplanted mice.

*Results: The proportion of CD19+Tim-1+ Regulatory B cells (Bregs) was significantly increased after the treatment with Iguratimod along with a higher level of IL-10. Allografts from Iguratimod-treated recipients showed significantly lower levels of IL-17A expression and higher levels of Foxp3 expression after both the first and the second skin transplantation, compared with control group. Flow cytometry of splenocytes from recipients confirmed that Iguratimod can reduce the proportion of Th17 cells and increases the proportion of Treg cells. Furthermore, intervention of Iguratimod remarkably reduced the levels of DSAs, increased the proportion of Bregs and the expression of IL-10 in vitro. Also, comparing to the solely Bregs infusion, we observed significant imbalance of Th17/Treg in co-intervention of Iguratimod and Bregs, indicating the Th17/Treg imbalance maybe the critical mechanism involved in Iguratimod efficacy which was modulated by Bregs. Blocking the IL-17 receptor with the monoclonal antibody suggested that the IL-17/IL-17R axis may be significantly correlated with the reduction of DSAs induced by Iguratimod.

*Conclusions: In conclusion, our study observed that Iguratimod can attenuate the production of DSAs by increasing the proportion of Bregs and then inducing the imbalance of Th17/Treg cells through IL-17/IL-17R axis.

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