ABSTRACT Cytokine reporter mice have been used to detect the in situ expression of cytokines via the coordinated expression of a reporter gene, such as a fluorescent protein, whenever the cytokine is transcribed. These reagents have led to critical advances in our understanding of cytokine regulation and physiology. This grant proposal focuses on the cytokine Interleukin 17D (IL-17D) based on our studies demonstrating that IL-17D is required for optimal anti-tumor, anti-viral, and anti-bacterial responses. In addition, IL-17D protects for age- associated dermatitis that could result of increased basophil activity. To better understand the physiologic role of IL-17D, it is critical to characterize its expression pattern in vivo. Although IL-17D expression can be detected by transcript in various tissues, this expression pattern cannot be easily confirmed using antibody-based approaches since the current antibodies to IL-17D also stain tissues in IL-17D-deficient animals. Therefore, we propose to further elucidate how IL-17D mediates its plethora of activities by generating IL-17D reporter animals via a CRISPR (clustered regularly interspaced short palindromic repeats)-based procedure using already validated constructs. In Aim 1, we will characterize these reporter animals at steady state and document the cellular and tissue expression of IL-17D using a combination of flow cytometry, immunofluorescence, and single cell RNA sequencing. These techniques will be optimized and further applied to Aim 2, where we begin to dissect IL-17D expression in mice challenged with viruses, bacteria, or allergen. Having already generated reporter cell lines using CRISPR, we propose in Aim 3 to identify key regulators of IL-17D by screening compounds in vitro on the reporter cell lines and validating these findings in vivo using reporter animals. Altogether, the proposed studies will provide necessary inroads for future studies dissecting the physiologic role of IL-17D in health and immunity.