Project Summary Regulatory T cells (Tregs) are a key subset of T cells important for the regulation of adaptive immune responses. Loss of function mutations in the Foxp3 gene, as seen in Scurfy murine strains and humans with immune dysregulation, polyendocrinopathy, and X-linked (IPEX) syndrome, lead to a systemic and fatal T cell-dependent lymphoproliferative disease, thus emphasizing the importance of Tregs in maintaining immune homeostasis. Given that Tregs are important in limiting overexcessive immune responses, understanding the biological signals that drive Treg function and expansion is key, especially in autoimmune and transplant settings in which modulation of Tregs could be beneficial for resolution of those disease states. Previously published work from our laboratory has shown that TCR signaling is critical for maintaining optimal Treg suppressive function and proliferation. In addition to TCR-mediated signals, our lab has shown that co-stimulation is also important in modulation of Treg expansion and persistence. While insight into the molecular cues that drive Treg biology has been slowly emerging, harnessing these cues to utilize Tregs as a potential therapy for the treatment of chronic inflammatory diseases still poses significant challenges. The expansion of polyclonal Tregs has been shown to ameliorate autoimmunity in mouse model, the suppression is not antigen- specific. Furthermore, it is technically challenging to selectively provide co-stimulatory signals only to Tregs and not to Tconvs. The main goal of this proposal is to utilize chimeric antigen receptors (CAR) to endow Treg with antigen specificity and enhance Treg function and expansion through the addition of co-stimulatory domains in the constructs. We will identify whether T cells transduced with CD19- CAR in conjunction with Foxp3 (CART19-Foxp3) will suppress antigen-specific conventional T cell (Tconv) proliferation in a B cell-dependent manner without CD19-specific killing of B cells by CART19-Foxp3 cells. Additionally, we will determine how the presence of co-stimulatory domains in the CAR construct influences CART19-Foxp3 function, persistence, and expansion. Furthermore, we will determine whether CART19-Foxp3 cells can, in turn, suppress B cell responses, such as antibody production, in vivo by modulation of conventional T cell activation. Further application of the studies in this aim will be extended to identifying the potential therapeutic use of CART-Foxp3 cells in treating chronic inflammation seen in a lupus-like model of chronic graft versus host (GVHD) that is dependent on autoantibody production by B cells. Through the proposed research plan, we will provide proof-of-principle that CAR- expressing Tregs suppress immune responses in an antigen-specific manner, which could be used as a novel therapeutic strategy for treatment of inflammatory conditions.