In vivo detection of immune responses via cytokine activity labeling.

While much is known about the identity and regulation of cytokine-producing cells, the cell types that respond to cytokines remain largely uncharacterized. To address this knowledge gap, we developed "cytokine cellular locating platforms" (CyCLoPs), a reporter system that translates cytokine receptor engagement into a genetically traceable signal. In vitro, CyCLoPs demonstrated high specificity, robust signal-to-background ratios, and broad applicability for probing diverse cytokine receptor interactions. In vivo, interleukin (IL)-17A-CyCLoPs reporter mice enabled the identification of IL-17A-responsive intestinal epithelial cells predominantly localized in the ileal villi following commensal bacterial colonization. Interferon-gamma (IFN-γ)-CyCLoPs reporter mice allowed for the detection of IFN-γ-exposed CD8(+) T cells within tumors, which expressed CD36, CD38, and leptin receptor and displayed gene signatures associated with reduced effector function. Collectively, CyCLoPs offers a platform for the direct visualization and characterization of cytokine-induced cellular responses and provides a tool for investigating how cytokines orchestrate distinct immunological outcomes in health and disease.