PROJECT SUMMARY/ABSTRACT Chronic intestinal inflammation is a major risk factor for the development of colorectal cancer (CRC). It is well- acknowledged that pro-inflammatory cytokines derived from immune cells play critical roles in almost every developmental stage of inflammation-induced cancers including CRC; however, molecular and cellular mechanisms underlying chronic intestinal inflammation leading to tumorigenesis are still being uncovered. Thus, understanding the mechanisms that control inflammatory responses of mucosal immune cells can be beneficial for the treatment of inflammation-associated CRC. Our preliminary results have revealed the potential role of E3 ligase, gene related to anergy in lymphocyte (Grail), in maintaining intestinal homeostasis and controlling carcinogenesis. Studies in both CRC murine models and patients show a correlation between Grail expression in colon and reduction of intestinal inflammation and tumor growth, suggesting the role of Grail in controlling intestinal inflammation and importantly projecting Grail as a potential marker for diagnosis of CRC. We detected more severe CRC growth in Grail knockout (KO) mice compared to wild-type (WT) mice, which was associated with increased expression of immune cell derived pro-inflammatory cytokines in the colon. Moreover, Grail expression in hematopoietic cells, particular in T cells, is important to control CRC. Remarkably, we detected a higher number of interleukin (IL)-17 producing T follicular helper (Tfh)17 cells in colons of tumor-bearing Grail KO mice and their percentage positively correlated with cancer severity. Similarly, we detected a higher number of Grail-insufficient Tfh17 cells in patients with advanced CRC compared to patients with early stage of disease, suggesting a new role for Tfh17 cells in promoting intestinal chronic inflammation and neoplastic diseases. Interestingly, elevated numbers of Tfh17 cells in Grail KO mice were associated with the accumulation of Th17 and B cells, indicating that Tfh cells could engage in crosstalk with other pro-carcinogenic immune cells. We propose a central hypothesis that Grail maintains intestinal homeostasis by controlling the activity of pro-inflammatory mucosal Tfh17 cells. In Aim 1, we will determine the link between Grail expression in Tfh cells and intestinal tumorigenesis. In Aim 2, we will examine the mechanism(s) whereby Grail deficient Tfh17 cells contribute to colon cancer development. In both aims, we will utilize gene KO approaches and Bcl6/IL17/IFN-reporter mice. The implication from this work is significant since it will provide novel insights into the genetic link between inflammation and cancer, define a new marker for early detection of inflammation-associated CRC, and guide the development of effective colon cancer immunotherapies.