Controlling tumor-infiltrating regulatory T cell function by the IL-17-Ybx1 signaling pathway

About This Grant

Project Summary/Abstract: Regulatory T cells (Tregs) play key roles in immune homeostasis, tumor development, and cancer therapy through suppression of inflammation. How the inflammatory tumor environment shapes the behavior of Tregs in turn, is less known. This knowledge gap precludes our ability to selectively block the activity of tumor infiltrating Tregs, while leaving systemic Treg function intact. One of the prominent inflammatory pathways involves interleukin-17 (IL-17), which has been shown to promote tumor development in multiple tissues including the gut. IL-17 family cytokines promote inflammation that drives the development of colorectal neoplasia, which eventually lead to colorectal cancer (CRC). Thus far, the underlying mechanism has largely been attributed to IL-17’s impact on tumor cells and myeloid cells. Our preliminary studies show that targeted ablation of IL-17 signaling on Treg cells increased colonic tumor development in mice, demonstrating a previously unknown protective role of IL-17 in CRC. We found that IL-17 directly signals to Tregs to promote their immune suppressive signature and more importantly, alternative splicing of RNAs in these cells. We also found that IL-17 promotes the expression of the RNA binding protein (RBP), Ybx1, and enhances the splicing of RNAs whose protein products are critical for Treg function. These genes (Foxp3, IL-27RA, and Stat3) also contain RNA motifs that can be recognized by Ybx1, whose role in Treg is unknown. Ablation of Ybx1 in both human and mouse Tregs resulted in reduced Treg immune suppression function, and reduced expression of immune suppressive protein Galactin-1. We demonstrate that proper splicing of Galactin-1 mRNA depends on Ybx1. These findings lead to our hypothesis that IL-17 promotes Treg function and suppresses CRC development through Ybx1 mediated alternative RNA splicing, and that a similar IL-17-Ybx1 pathway promotes Treg activity in established tumors and impedes cancer immunotherapy. Given the critical roles of both Tregs and Th17 cells in tumor development and cancer therapy, along with the knowledge gap relating to the impact of RBPs on Treg biology, we propose the following studies: 1) delineate the mechanism by which the IL-17-Ybx1 pathway regulates alternative RNA splicing and function of Tregs; 2) test if the IL-17-Ybx1 pathway in CRC can be manipulated to prevent gut tumorigenesis; and 3) test the feasibility of targeting IL-17-Ybx1 pathway for the enhancement of cancer immunotherapy. These investigations will provide new insights into the mechanisms by which the inflammatory tumor microenvironment (TME) controls Treg function, and guide the invention and use of novel therapies for the treatment of CRC in humans. For example, based on a role for Ybx1 in promoting immune suppressive function of Tregs in CRC, we may employ targeted therapy by degrading Ybx1 in tumor infiltrating Tregs to foster an immunologically “hot” environment for enhanced cancer immunotherapy. Justification for the use of animal models We propose to use multiple animal models of cancers in this project. These models closely mimic human colorectal cancers by simulating tumor cell mutation process, microbiota interaction, and tumor environment evolution, allowing us to interrogate the mechanism of Ybx1-controlled Treg cell function in cancers. These models cannot be replaced with non-animal studies such as in vitro cell culture or organs-on-a-chip systems.