PROJECT SUMMARY/ABSTRACT Understanding peripheral tolerance and the maintenance of immune system homeostasis are vital in the control of human diseases. We have previously demonstrated that anti-inflammatory cytokine IL-37 participates in immune tolerance by generating semi-mature tolerogenic dendritic cells (DCs) in antigen- specific adaptive immune responses. IL-37 is one of eleven IL-1 family members and the only known member to be broadly anti-inflammatory. In our recent project, we found IL-37 levels were elevated in multiple human immune cell types, specifically in regulatory T cells (Tregs) cells. Further analysis revealed that human Treg cells express the highest IL-37 levels among all T-cell subsets and that intracellular expression of IL-37 correlates with the expression of master transcriptional regulator, FOXP3, in human Treg cells. Our current project hypothesizes that elevated IL-37 expression stabilizes Treg cells and induces potent immune suppression by controlling FOXP3 expression. IL-37 is not expressed in mice, but using transgenic mice and peripheral blood T cells from human donors, we generated strong preliminary data to support our hypothesis. In this grant proposal, we will use human primary Treg cells and T cell lines overexpressing IL-37 and its mutant form to elucidate the biological and molecular mechanisms of IL-37 in controlling human Treg cell function. Since our proposal uses human T cells, the results could be easily translated into clinical medicine and patient care. Our findings will have an immense translational impact on many human diseases such as autoimmunity and transplantation.