*Purpose: Soluble fibrinogen-like protein 2 (sFGL2) is a novel immunoregulatory molecule, secreted mainly by regulatory T cells. CD11b⁺ Gr1⁺ myeloid-derived suppressor cells (MDSCs) are an important regulatory innate cell population and have significant inhibitory effect on T cell-mediated responses. Here, we synthesized murine full length sFGL2 by eukaryotic expression system, and investigated the impact on differentiation and function of MDSCs.

*Methods: Bone marrow cells from BABL/c mice were cultured with or without 10 μg/ml sFGL2 for 3 days and 5 days under 10 ng/ml GM-CSF stimulation. MDSC phenotypes were analyzed by flow cytometry. The mRNA and protein level of arginase-1 and iNOS was tested by q-PCR and western blot. To evaluate the sFGL-2-induced MDSC function of immunosuppression in vivo, cells were infusion via tail vein into mice using skin transplant model.

*Results: Compared with PBS, sFGL2 significantly induced CD11b⁺Ly6G^–Ly6C^high MDSC (MO-MDSC) differentiation but inhibited CD11b⁺Ly6G⁺Ly6C^low MDSC (PMN-MDSC) differentiation. The sFGL2-induced MO-MDSCs significantly inhibited T cells proliferation compared with those induced by PBS. Besides, sFGL2-induced MO-MDSCs demonstrated higher expression of arginase-1 and iNOS at both mRNA and protein level. Furthermore, adoptive transfer sFGL2-induced MO-MDSCs prolonged the skin allograft survival in mice. In the sFGL2-induced MO-MDSCs infusion group, the transplanted skin allograft showed mild inflammatory immune cell infiltration, less apoptosis and necrosis, and lower pro-inflammatory cytokines expression. T cells in the recipient mouse displayed a lower autoimmune phenotype (lower TCR⁺ CD44^high CD62^low cells).

*Conclusions: Taken together, our results indicate sFGL2 prompts MO-MDSCs differentiation and enhances their immunosuppressive function.

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