Super-silent FRET Sensor Enables Live Cell Imaging and Flow Cytometric Stratification of Intracellular Serine Protease Activity in Neutrophils

Serine proteases are released by neutrophils to act primarily as antimicrobial proteins but excessive and unbalanced serine protease activity results in serious host tissue damage. Here the synthesis of a novel chemical sensor based on a multi-branched fluorescence quencher is reported. It is super-silent, exhibiting no fluorescence until de-quenched by the exemplar serine protease human neutrophil elastase, rapidly enters human neutrophils, and is inhibited by serine protease inhibitors. This sensor allows live imaging of intracellular serine protease activity within human neutrophils and demonstrates that the unique combination of a multivalent scaffold combined with a FRET peptide represents a novel and efficient strategy to generate super-silent sensors that permit the visualisation of intracellular proteases and may enable point of care whole blood profiling of neutrophils.