Exploring Mechanisms of Regulatory T cell-mediated Immune Suppression and Development of Anti-PD-L1 CAR T cells

Regulatory T cells (Tregs) suppress adaptive immunity and inflammation. In cancer, Tregs hinder therapeutic responses due to suppression of anti-tumor activity in the tumor microenvironment. Although these cells play a role in suppressing anti-tumor responses, development of therapeutics that target Tregs is limited by their low abundance, heterogeneity, and lack of specific cell surface markers. To study Treg mechanisms of suppression, a human T cell line, MoT, was identified and characterized as a model of human Foxp3+ Tregs. MoT cells express surface markers consistent with PBMC-derived Tregs and inhibit proliferation of CD4+ responder PBMCs in a ratio-dependent manner. Transwell membrane separation prevented suppression of stimulated CD4+ PBMC proliferation by MoT cells, suggesting cell-cell contact is required for suppressive activity. Suppression was found to be independent of soluble cytokines and known immune checkpoint pathways, providing evidence that a Foxp3+ Treg population suppresses immune responses by an unknown cell contact-dependent mechanism. To investigate potential cell surface molecules that mediate suppression, monoclonal antibodies (mAbs) were generated to a known immunosuppressive protein, Galectin-1, and to MoT cell surface proteins. MAbs were identified that bind and functionally block suppressive activity. Another mechanism of immune suppression involves the PD-1/PD-L1 pathway, which is exploited by tumor cells to resist T cell killing and escape immune clearance. Since PD-L1 has emerged as an effective therapeutic target, anti-PD-L1 CAR T cells were generated and demonstrated to kill PD-L1-positive tumor cells. These results expand upon the current knowledge of Treg function and CAR T cell therapy and may lead to enhanced anti-tumor immunity to improve patient responses.