A genetically encoded sensor for visualizing leukotriene B4 gradients in vivo

Leukotriene B 4 (LTB 4 ) is a potent lipid chemoattractant driving inflammatory responses during host defense, allergy, autoimmune and metabolic diseases. Gradients of LTB 4 orchestrate leukocyte recruitment and swarming to sites of tissue damage and infection. How LTB 4 gradients form and spread in live tissues to regulate these processes remains largely elusive due to the lack of suitable tools for monitoring LTB 4 levels in vivo. Here, we develop GEM-LTB 4 , a genetically encoded green fluorescent LTB 4 biosensor based on the human G-protein-coupled receptor BLT1. GEM-LTB 4 shows high sensitivity, specificity and a robust fluorescence increase in response to LTB 4 without affecting downstream signaling pathways. We use GEM-LTB 4 to measure ex vivo LTB 4 production of murine neutrophils. Transgenic expression of GEM-LTB 4 in zebrafish allows the real-time visualization of both exogenously applied and endogenously produced LTB 4 gradients. GEM-LTB 4 thus serves as a broadly applicable tool for analyzing LTB 4 dynamics in various experimental systems and model organisms.