Abstract
FoxP3+ regulatory T cells (Tregs) are responsible for immune homeostasis by suppressing excessive anti-self-immunity. Tregs facilitate tumor growth by inhibiting anti-tumor immunity. Here, we explored the targeting of FoxP3 as a basis for new immunotherapies. In a high-throughput phenotypic screening of a drug repurposing library using human primary T cells, we identified quinacrine as a FoxP3 downregulator. In silico searches based on the structure of quinacrine, testing of sub-libraries of analogs in vitro, and validation identified a subset of 9-amino-acridines that selectively abrogated Treg suppressive functions. Mechanistically, these acridines interfered with the DNA-binding activity of FoxP3 and inhibited FoxP3-regulated downstream gene regulation. Release from Treg suppression by 9-amino-acridines increased anti-tumor immune responses both in cancer patient samples and in mice in a syngeneic tumor model. Our study highlights the feasibility of screening for small molecular inhibitors of FoxP3 as an approach to pursuing Treg-based immunotherapy.