Expression of IL-37 contributes to the immunosuppressive property of human CD4+CD25+ regulatory T cells

Interleukin-37 (IL-37) possesses the function of down-regulate systemic and local inflammation. It is unknown whether IL-37 is expressed in human regulatory T cells (Tregs) and its role in modulating the immune response of Tregs. In the present study, cell surface molecules and secretory cytokines were analyzed in order to determine the function of IL-37 in regulating inhibitory effect of human CD4(+)CD25(+)Tregs. Meanwhile, the effects of IL-37 on T cell differentiation and proliferation as co-culture of CD4(+)CD25(+)Treg/CD4(+)CD25(-)T cell were also investigated. It was showed that IL-37 was expressed in cytoplasm of CD4(+)CD25(+)Tregs, and the levels of IL-37 were gradually elevated with the enhanced activity of CD4(+)CD25(+)Tregs. Secretory cytokines such as transforming growth factor (TGF)-β and interleukin (IL)-10, and expressions of cell surface molecules, including forkhead/winged helix transcription factor p3 (FOXP3) and cytotoxic T-lymphocyte associated antigen (CTLA)-4, were significantly decreased when IL-37 gene was silenced by siRNA. Furthermore, down-regulation of IL-37 expression in human CD4(+)CD25(+)Tregs obviously promoted proliferation of co-cultured T cell and differentiation, together with observably enhancement of IL-2 formation. These results demonstrated that IL-37 might manifest as a critical protein involving in immunosuppression of human CD4(+)CD25(+)Tregs.