Abstract 3850: FOXP3 dimerization by stapled alpha-helical peptides alters regulatory T cell function
Authors: Eclov Rachel , Fefferman Marie , Samaeekia Ravand , LaBelle James L. 
Published: August 2018

Abstract
Regulatory T cells (Tregs) are critical for the maintenance of immune tolerance and control of immune responses to foreign agents. Tregs depend upon the transcriptional protein FOXP3 for their ontogeny and genetic orchestration of their suppressive mechanisms. While T cells have the ability to target tumor-associated antigens from many cancer subtypes, these T cells are often suppressed in patients with immunologically “silenced” tumors. The tolerizing function of Tregs can limit effective anti-tumor T cell-mediated killing, even in the presence of immunotherapies, including checkpoint inhibition, and their presence predicts reduced survival in patients with a myriad of cancers. Currently, there exists no means of depleting Tregs in patients without also targeting activated effector T cells. In an effort to overcome this, we have specifically inhibited Tregs by targeting FOXP3 through molecular mimicry using peptides designed to inhibit FOXP3 homodimerization. Homodimerization of FOXP3 is mediated by an antiparallel coiled-coil interaction between the leucine-zipper domains of two FOXP3 proteins. A series of single and double hydrocarbon-stapled alpha-helical (SAH) peptides were synthesized using the leucine zipper dimerization domain (DD) interface of FOXP3 (SAH-FOXP3DDs) as a molecular template. SAH-FOXP3DDs, but not native peptides, specifically bind FOXP3's leucine zipper domain with nanomolar affinity. Unlike stapled peptide point mutant controls, lead SAH-FOXP3DDs also showed dose-dependent inhibition of FOXP3 binding to cognate DNA. Lead SAH-FOXP3DDs are cell permeable and nontoxic to T cells as measured by absence of non-specific LDH-release and apoptosis induction following treatment. Flow cytometric analysis of treated Tregs measured changes in expression of a number of Treg/T cell - associated proteins including increases in FOXP3 and CD127 and decreases in CD25, while no changes were observed in treated conventional CD4+ T cells (Tcons) or with cells treated with point-mutant control peptides. In addition, treated Tregs demonstrated mRNA expression changes of FOXP3-regulated genes including Foxp3, CD127, IL2 and Ctla. Treatment of freshly isolated and in vitro expanded Tregs with lead SAH-FOXP3DD compounds inhibited Treg-mediated suppression of Tcon proliferation in response to CD3/CD28 activation, while no effects were measured with a point-mutant control. We believe that SAH-FOXP3DDs will be powerful molecular probes to further our understanding of FOXP3 transcriptional control. Additionally, they may be a foundation for prototype therapeutics to target Tregs in cancer patients in an effort to amplify the endogenous anti-tumor immune response alone or in combination with other cancer immunotherapies.
Citation Format: Rachel Eclov, Marie Fefferman, Ravand Samaeekia, James L. LaBelle. FOXP3 dimerization by stapled alpha-helical peptides alters regulatory T cell function [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3850.