Application of activity-based probes to detect intra- and extracellular cathepsin G activity in immune cells

Immune cells are critical in diverse defense mechanisms, including elimination of viral infected- and tumor cells as well as maintaining tolerance. It was demonstrated that cathepsin G (CatG), a serine protease, binds to the cell surface of natural killer cells, T cells, B cells, and activated monocytes; however, the distribution of cell surface and intracellular CatG activity of immune cell subsets has not been identified. To this end, we analyzed CatG activity in several immune cells by flow cytometry and found that levels of CatG differs between immune cells, which was determined on the protein- and activity level by using a specific CatG antibody and activity-based probes (MARS116). Thus, MARS116 is a novel reporter of cell surface and intracellular CatG activity and can be used to differentiate between distinct immune cells.