Abstract In case of radiation accidents or nuclear warfare, exposure of the small intestine and colon to high-dose irradiation results in delayed injury, manifested by an increase in fibrosis, that significantly impacts organ function. There are no current FDA-approved medical countermeasures to prevent, mitigate, or treat delayed radiation injury of the small intestine and colon. However, one major challenge for the development of such therapies is to understand how fibrosis associated with DEARE (Delayed Effects of Acute Radiation Exposure) is impacted by biological sex, as recent studies suggest that females may be more sensitive to delayed radiation-induced fibrosis compared to males. Our long-term goal is to uncover the mechanisms underlying the effects of biological sex on acute and long-term radiation injury of the GI tract. Type I IFN has been proposed as a potential mitigator of acute radiation injury to the intestine by promoting epithelial regeneration. Furthermore, Type I IFN is activated in the intestine and colon following acute radiation injury and is persistently upregulated in our model of DEARE. In preliminary studies, we evaluated radiation-induced delayed colonic fibrosis in male and female Irgm1-/- mice that are known to potently activate Type I IFN and found that germline Irgm1 deletion results in substantially greater colonic fibrosis in females compared to males. Therefore, in this application, we hypothesize that Type I Interferon signaling, while preventing acute radiation injury to the GI tract, promotes long-term intestinal fibrosis to a greater degree in females vs. males. We will test this hypothesis through the following Specific Aims: Aim 1: To determine the molecular mechanisms of sex-dependent, Type I Interferon signaling in radiation-induced fibrosis. Aim 2: To determine the cellular origins of sex-dependent, radiation-induced fibrosis. Aim 3: To determine the efficacy of FDA approved Type I Interferon inhibitors to prevent the development of radiation-induced intestinal fibrosis in females vs. males. Together, these studies will reveal novel mechanisms by which activation of Type I Interferon promotes the development of chronic submucosal fibrosis in the gastrointestinal tract in sex- dependent manner, which is a major complication of survival from gastrointestinal acute radiation syndrome (GI-ARS). These results will inform the future development of mitigators of DEARE in female and male populations.