Abstract LB-B20: High-affinity antisense oligonucleotides targeting Foxp3 inhibit immunosuppressive function of regulatory T cells and produce anti-tumor effects in syngeneic tumor models

Regulatory T cells (Treg) contribute to the progression of cancer through suppression of specific anti-tumor effector immune responses. Therefore, inhibition of Treg function is an attractive approach for cancer immunotherapy. However, despite substantial effort, specific inhibition of Tregs remains a challenge. Foxp3 is a Treg-specific transcription factor required for their development and function. We employed next-generation antisense inhibitors (Gen 2.5 cEt-modified ASOs) to selectively inhibit Foxp3 expression in mouse Treg cells and evaluated consequences of ASO-mediated Foxp3 knock-down in vitro and in vivo. Mouse Foxp3-specific ASOs produced potent dose-dependent reductions in Foxp3 mRNA and protein in mouse Tregs in vitro without the use of transfection reagents. Reductions of Foxp3 in mouse Tregs was associated with loss of expression of immunosuppressive markers as well as loss of immunosuppressive function in vitro. Systemically delivered unformulated cEt-modified mouse Foxp3 ASOs reduced Foxp3 mRNA and protein in Tregs in normal mice. Importantly, this inhibition, in contrast to the phenotype of Foxp3 KO mice, was not associated with a significant autoimmune phenotype. When mouse Foxp3 ASOs were administered to syngeneic tumor-bearing mice they produced significant Foxp3 mRNA and protein knockdown in tumor-infiltrating Tregs. The growth of tumors in mouse Foxp3 ASO-treated animals was significantly attenuated with a fraction of animals (25%-50%) achieving complete regressions. Anti-tumor activity of mouse Foxp3 ASOs was associated with immunophenotype changes consistent with the increased anti-tumor immune response. Overall these data demonstrate that regulatory T cells can be effectively targeted by ASOs and that Foxp3 ASOs represent a potentially attractive therapeutic approach in cancer immunotherapy. Citation Format: Alexey S. Revenko, Charles Sinclair, Ben Johnson, Lisa Hettrick, Alison Peter, Anna Staniszewska, Adina Hughes, Linda Sandin, Molly Taylor, Stephanie Klein, Andy Watt, Brett Monia, Mark Edbrooke, A. Robert MacLeod. High-affinity antisense oligonucleotides targeting Foxp3 inhibit immunosuppressive function of regulatory T cells and produce anti-tumor effects in syngeneic tumor models [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr LB-B20.